I think this may happen when a new version of a plugin is being registered. It may not actually have to be different from the db version of the plugin. I'm not sure.

Anyway, during plugin registration you get a failure (full trace attached) with a root cause like the following:

Caused by: org.hibernate.HibernateException: Javassist Enhancement failed: org.rhq.core.domain.plugin.Plugin
        at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.getProxy(JavassistLazyInitializer.java:118)
        at org.hibernate.proxy.pojo.javassist.JavassistProxyFactory.getProxy(JavassistProxyFactory.java:48)
        at org.hibernate.tuple.entity.AbstractEntityTuplizer.createProxy(AbstractEntityTuplizer.java:379)
        at org.hibernate.persister.entity.AbstractEntityPersister.createProxy(AbstractEntityPersister.java:3460)
        at org.hibernate.event.def.DefaultLoadEventListener.createProxyIfNecessary(DefaultLoadEventListener.java:257)
        at org.hibernate.event.def.DefaultLoadEventListener.proxyOrLoad(DefaultLoadEventListener.java:191)
        at org.hibernate.event.def.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:103)
        at org.hibernate.impl.SessionImpl.fireLoad(SessionImpl.java:878)
        at org.hibernate.impl.SessionImpl.load(SessionImpl.java:795)
        at org.hibernate.impl.SessionImpl.load(SessionImpl.java:788)
        at org.hibernate.ejb.AbstractEntityManagerImpl.getReference(AbstractEntityManagerImpl.java:153)
        ... 94 more
Caused by: java.lang.NullPointerException
        at javassist.util.proxy.RuntimeSupport$DefaultMethodHandler.invoke(RuntimeSupport.java:37)
        at org.rhq.core.domain.plugin.Plugin_$$_javassist_68.setDeployment(Plugin_$$_javassist_68.java)
        at org.rhq.core.domain.plugin.Plugin.<init>(Plugin.java:205)
        at org.rhq.core.domain.plugin.Plugin_$$_javassist_68.<init>(Plugin_$$_javassist_68.java)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
        at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
        at java.lang.Class.newInstance0(Class.java:355)
        at java.lang.Class.newInstance(Class.java:308)
        at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.getProxy(JavassistLazyInitializer.java:115)
        ... 104 more


I don't currently have any reproduction steps, but I've seen this more than once.  The server startup continues but is most likely invalid from that point on.  A subsequent restart may normalize the situation but I'm not sure.