Reproduces not all the time.
Sometime when trying to preview snapshot getting error and the following exception in the engine:
It might happen when clicking twice on preview button. There is some delay when the button becomes greyed out after clicking on it. There is also an error message in the GUI: "Error while executing action Preview Snapshot: internal oVirt Engine Error"
-------------------------

SCommand, return: f68d642b-b9fd-46fe-9d72-7dec5a533978, log id: 338732e8
2012-06-28 11:36:49,745 INFO  [org.ovirt.engine.core.bll.SPMAsyncTask] (ajp--0.0.0.0-8009-17) [4fbb1d50] BaseAsyncTask::AddOrUpdateTaskInDB: Adding task b9483255-d6ec-4cfb-b699-165582630386 to DataBase
2012-06-28 11:36:49,782 INFO  [org.ovirt.engine.core.bll.EntityMultiAsyncTasks] (ajp--0.0.0.0-8009-17) [4fbb1d50] EntityMultiAsyncTasks::AttachTask: Attaching task b9483255-d6ec-4cfb-b699-165582630386 to entity c544ec44-6ccc-4619-a276-a0f1efbdd92c.
2012-06-28 11:36:49,786 INFO  [org.ovirt.engine.core.bll.AsyncTaskManager] (ajp--0.0.0.0-8009-17) [4fbb1d50] AsyncTaskManager::AddTaskToManager: Adding task b9483255-d6ec-4cfb-b699-165582630386 (Parent Command TryBackToAllSnapshotsOfVm, Parameters Type org.ovirt.engine.core.common.asynctasks.AsyncTaskParameters), polling hasn't started yet..
2012-06-28 11:36:49,859 ERROR [org.ovirt.engine.core.bll.TryBackToAllSnapshotsOfVmCommand] (ajp--0.0.0.0-8009-18) [73451a8d] Command org.ovirt.engine.core.bll.TryBackToAllSnapshotsOfVmCommand throw exception: org.springframework.dao.DataIntegrityViolationException: CallableStatementCallback; SQL [{call insertsnapshot(?, ?, ?, ?, ?, ?, ?, ?)}]; ERROR: duplicate key value violates unique constraint "pk_snapshots"
  Where: SQL statement "INSERT INTO snapshots( snapshot_id, status, vm_id, snapshot_type, description, creation_date, app_list, vm_configuration) VALUES(  $1 ,  $2 ,  $3 ,  $4 ,  $5 ,  $6 ,  $7 ,  $8 )"
PL/pgSQL function "insertsnapshot" line 2 at SQL statement; nested exception is org.postgresql.util.PSQLException: ERROR: duplicate key value violates unique constraint "pk_snapshots"
  Where: SQL statement "INSERT INTO snapshots( snapshot_id, status, vm_id, snapshot_type, description, creation_date, app_list, vm_configuration) VALUES(  $1 ,  $2 ,  $3 ,  $4 ,  $5 ,  $6 ,  $7 ,  $8 )"
PL/pgSQL function "insertsnapshot" line 2 at SQL statement
        at org.springframework.jdbc.support.SQLErrorCodeSQLExceptionTranslator.doTranslate(SQLErrorCodeSQLExceptionTranslator.java:228) [spring-jdbc-2.5.6.SEC03.jar:2.5.6.SEC03]
        at org.springframework.jdbc.support.AbstractFallbackSQLExceptionTranslator.translate(AbstractFallbackSQLExceptionTranslator.java:72) [spring-jdbc-2.5.6.SEC03.jar:2.5.6.SEC03]
        at org.springframework.jdbc.core.JdbcTemplate.execute(JdbcTemplate.java:952) [spring-jdbc-2.5.6.SEC03.jar:2.5.6.SEC03]
        at org.springframework.jdbc.core.JdbcTemplate.call(JdbcTemplate.java:985) [spring-jdbc-2.5.6.SEC03.jar:2.5.6.SEC03]
        at org.springframework.jdbc.core.simple.AbstractJdbcCall.executeCallInternal(AbstractJdbcCall.java:368) [spring-jdbc-2.5.6.SEC03.jar:2.5.6.SEC03]
        at org.springframework.jdbc.core.simple.AbstractJdbcCall.doExecute(AbstractJdbcCall.java:342) [spring-jdbc-2.5.6.SEC03.jar:2.5.6.SEC03]
        at org.springframework.jdbc.core.simple.SimpleJdbcCall.execute(SimpleJdbcCall.java:164) [spring-jdbc-2.5.6.SEC03.jar:2.5.6.SEC03]
        at org.ovirt.engine.core.dal.dbbroker.SimpleJdbcCallsHandler.executeImpl(SimpleJdbcCallsHandler.java:124) [engine-dal.jar:]
        at org.ovirt.engine.core.dal.dbbroker.SimpleJdbcCallsHandler.executeModification(SimpleJdbcCallsHandler.java:37) [engine-dal.jar:]
        at org.ovirt.engine.core.dao.DefaultGenericDaoDbFacade.save(DefaultGenericDaoDbFacade.java:93) [engine-dal.jar:]
        at org.ovirt.engine.core.bll.snapshots.SnapshotsManager.addSnapshot(SnapshotsManager.java:132) [engine-bll.jar:]
        at org.ovirt.engine.core.bll.snapshots.SnapshotsManager.addSnapshot(SnapshotsManager.java:95) [engine-bll.jar:]
        at org.ovirt.engine.core.bll.TryBackToAllSnapshotsOfVmCommand.ExecuteVmCommand(TryBackToAllSnapshotsOfVmCommand.java:90) [engine-bll.jar:]
        at org.ovirt.engine.core.bll.VmCommand.executeCommand(VmCommand.java:100) [engine-bll.jar:]
        at org.ovirt.engine.core.bll.CommandBase.ExecuteWithoutTransaction(CommandBase.java:804) [engine-bll.jar:]
        at org.ovirt.engine.core.bll.CommandBase.executeActionInTransactionScope(CommandBase.java:896) [engine-bll.jar:]
        at org.ovirt.engine.core.bll.CommandBase.runInTransaction(CommandBase.java:1200) [engine-bll.jar:]
        at org.ovirt.engine.core.utils.transaction.TransactionSupport.executeInNewTransaction(TransactionSupport.java:204) [engine-utils.jar:]
        at org.ovirt.engine.core.utils.transaction.TransactionSupport.executeInRequired(TransactionSupport.java:142) [engine-utils.jar:]
        at org.ovirt.engine.core.utils.transaction.TransactionSupport.executeInScope(TransactionSupport.java:109) [engine-utils.jar:]
        at org.ovirt.engine.core.bll.CommandBase.Execute(CommandBase.java:911) [engine-bll.jar:]
        at org.ovirt.engine.core.bll.CommandBase.ExecuteAction(CommandBase.java:268) [engine-bll.jar:]
        at org.ovirt.engine.core.bll.Backend.runActionImpl(Backend.java:336) [engine-bll.jar:]
        at org.ovirt.engine.core.bll.Backend.RunAction(Backend.java:294) [engine-bll.jar:]
        at sun.reflect.GeneratedMethodAccessor132.invoke(Unknown Source) [:1.6.0_24]
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) [rt.jar:1.6.0_24]
        at java.lang.reflect.Method.invoke(Method.java:616) [rt.jar:1.6.0_24]
        at org.jboss.as.ee.component.ManagedReferenceMethodInterceptorFactory$ManagedReferenceMethodInterceptor.processInvocation(ManagedReferenceMethodInterceptorFactory.java:72) [jboss-as-ee.jar:7.1.1.Final-redhat-1]
        at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation.jar:1.1.1.Final-redhat-1]
        at org.jboss.invocation.InterceptorContext$Invocation.proceed(InterceptorContext.java:374) [jboss-invocation.jar:1.1.1.Final-redhat-1]
        at org.ovirt.engine.core.utils.ThreadLocalSessionCleanerInterceptor.injectWebContextToThreadLocal(ThreadLocalSessionCleanerInterceptor.java:11) [engine-utils.jar:]
        at sun.reflect.GeneratedMethodAccessor10.invoke(Unknown Source) [:1.6.0_24]
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) [rt.jar:1.6.0_24]
        at java.lang.reflect.Method.invoke(Method.java:616) [rt.jar:1.6.0_24]
        at org.jboss.as.ee.component.ManagedReferenceLifecycleMethodInterceptorFactory$ManagedReferenceLifecycleMethodInterceptor.processInvocation(ManagedReferenceLifecycleMethodInterceptorFactory.java:123) [jboss-as-ee.jar:7.1.1.Final-redhat-1]
        at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation.jar:1.1.1.Final-redhat-1]
        at org.jboss.invocation.WeavedInterceptor.processInvocation(WeavedInterceptor.java:53) [jboss-invocation.jar:1.1.1.Final-redhat-1]
        at org.jboss.as.ee.component.interceptors.UserInterceptorFactory$1.processInvocation(UserInterceptorFactory.java:36) [jboss-as-ee.jar:7.1.1.Final-redhat-1]
        at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation.jar:1.1.1.Final-redhat-1]
        at org.jboss.invocation.InitialInterceptor.processInvocation(InitialInterceptor.java:21) [jboss-invocation.jar:1.1.1.Final-redhat-1]
        at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation.jar:1.1.1.Final-redhat-1]
        at org.jboss.invocation.ChainedInterceptor.processInvocation(ChainedInterceptor.java:61) [jboss-invocation.jar:1.1.1.Final-redhat-1]
        at org.jboss.as.ee.component.interceptors.ComponentDispatcherInterceptor.processInvocation(ComponentDispatcherInterceptor.java:53) [jboss-as-ee.jar:7.1.1.Final-redhat-1]
        at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation.jar:1.1.1.Final-redhat-1]
        at org.jboss.as.ejb3.component.singleton.SingletonComponentInstanceAssociationInterceptor.processInvocation(SingletonComponentInstanceAssociationInterceptor.java:53) [jboss-as-ejb3.jar:7.1.1.Final-redhat-1]
        at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation.jar:1.1.1.Final-redhat-1]
        at org.jboss.as.ejb3.tx.CMTTxInterceptor.invokeInNoTx(CMTTxInterceptor.java:211) [jboss-as-ejb3.jar:7.1.1.Final-redhat-1]
        at org.jboss.as.ejb3.tx.CMTTxInterceptor.supports(CMTTxInterceptor.java:363) [jboss-as-ejb3.jar:7.1.1.Final-redhat-1]
        at org.jboss.as.ejb3.tx.CMTTxInterceptor.processInvocation(CMTTxInterceptor.java:194) [jboss-as-ejb3.jar:7.1.1.Final-redhat-1]
        at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation.jar:1.1.1.Final-redhat-1]
        at org.jboss.as.ejb3.component.interceptors.CurrentInvocationContextInterceptor.processInvocation(CurrentInvocationContextInterceptor.java:41) [jboss-as-ejb3.jar:7.1.1.Final-redhat-1]
        at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation.jar:1.1.1.Final-redhat-1]
        at org.jboss.as.ejb3.component.interceptors.LoggingInterceptor.processInvocation(LoggingInterceptor.java:59) [jboss-as-ejb3.jar:7.1.1.Final-redhat-1]
        at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation.jar:1.1.1.Final-redhat-1]
        at org.jboss.as.ee.component.NamespaceContextInterceptor.processInvocation(NamespaceContextInterceptor.java:50) [jboss-as-ee.jar:7.1.1.Final-redhat-1]
        at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation.jar:1.1.1.Final-redhat-1]
        at org.jboss.as.ee.component.TCCLInterceptor.processInvocation(TCCLInterceptor.java:45) [jboss-as-ee.jar:7.1.1.Final-redhat-1]
        at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation.jar:1.1.1.Final-redhat-1]
        at org.jboss.invocation.ChainedInterceptor.processInvocation(ChainedInterceptor.java:61) [jboss-invocation.jar:1.1.1.Final-redhat-1]
        at org.jboss.as.ee.component.ViewService$View.invoke(ViewService.java:165) [jboss-as-ee.jar:7.1.1.Final-redhat-1]
        at org.jboss.as.ee.component.ViewDescription$1.processInvocation(ViewDescription.java:173) [jboss-as-ee.jar:7.1.1.Final-redhat-1]
        at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288) [jboss-invocation.jar:1.1.1.Final-redhat-1]
        at org.jboss.invocation.ChainedInterceptor.processInvocation(ChainedInterceptor.java:61) [jboss-invocation.jar:1.1.1.Final-redhat-1]
        at org.jboss.as.ee.component.ProxyInvocationHandler.invoke(ProxyInvocationHandler.java:72) [jboss-as-ee.jar:7.1.1.Final-redhat-1]
        at org.ovirt.engine.core.common.interfaces.BackendLocal$$$view8.RunAction(Unknown Source) [engine-common.jar:]
        at org.ovirt.engine.ui.frontend.server.gwt.GenericApiGWTServiceImpl.RunAction(GenericApiGWTServiceImpl.java:120)
        at sun.reflect.GeneratedMethodAccessor131.invoke(Unknown Source) [:1.6.0_24]
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) [rt.jar:1.6.0_24]
        at java.lang.reflect.Method.invoke(Method.java:616) [rt.jar:1.6.0_24]
        at com.google.gwt.rpc.server.RPC.invokeAndStreamResponse(RPC.java:196)
        at com.google.gwt.rpc.server.RpcServlet.processCall(RpcServlet.java:161)
        at com.google.gwt.rpc.server.RpcServlet.processPost(RpcServlet.java:222)
        at com.google.gwt.user.server.rpc.AbstractRemoteServiceServlet.doPost(AbstractRemoteServiceServlet.java:62)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:754) [jboss-servlet-api_3.0_spec.jar:1.0.0.Final-redhat-1]
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:847) [jboss-servlet-api_3.0_spec.jar:1.0.0.Final-redhat-1]
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:329)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:248)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:275)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:161)
        at org.jboss.as.web.security.SecurityContextAssociationValve.invoke(SecurityContextAssociationValve.java:153)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:155)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
        at org.jboss.web.rewrite.RewriteValve.invoke(RewriteValve.java:466)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:368)
        at org.apache.coyote.ajp.AjpProcessor.process(AjpProcessor.java:505)
        at org.apache.coyote.ajp.AjpProtocol$AjpConnectionHandler.process(AjpProtocol.java:445)
        at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:930)
        at java.lang.Thread.run(Thread.java:679) [rt.jar:1.6.0_24]
Caused by: org.postgresql.util.PSQLException: ERROR: duplicate key value violates unique constraint "pk_snapshots"
  Where: SQL statement "INSERT INTO snapshots( snapshot_id, status, vm_id, snapshot_type, description, creation_date, app_list, vm_configuration) VALUES(  $1 ,  $2 ,  $3 ,  $4 ,  $5 ,  $6 ,  $7 ,  $8 )"
PL/pgSQL function "insertsnapshot" line 2 at SQL statement
        at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:2062)
        at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:1795)
        at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:257)
        at org.postgresql.jdbc2.AbstractJdbc2Statement.execute(AbstractJdbc2Statement.java:479)
        at org.postgresql.jdbc2.AbstractJdbc2Statement.executeWithFlags(AbstractJdbc2Statement.java:367)
        at org.postgresql.jdbc2.AbstractJdbc2Statement.execute(AbstractJdbc2Statement.java:360)
        at org.jboss.jca.adapters.jdbc.CachedPreparedStatement.execute(CachedPreparedStatement.java:297)
        at org.jboss.jca.adapters.jdbc.WrappedPreparedStatement.execute(WrappedPreparedStatement.java:404)
        at org.springframework.jdbc.core.JdbcTemplate$5.doInCallableStatement(JdbcTemplate.java:987) [spring-jdbc-2.5.6.SEC03.jar:2.5.6.SEC03]
        at org.springframework.jdbc.core.JdbcTemplate.execute(JdbcTemplate.java:936) [spring-jdbc-2.5.6.SEC03.jar:2.5.6.SEC03]