The problem occurs in mock-0.7.1 on Fedora 7, in the do() method. It seems to me
that the return status of the child process created by popen2.Popen4() is never
assigned to retval before exiting. So default value zero is always returned to
the parent process, and the consequence is that build errors are ignored by
mock. Here is a patch that makes it work for me:

--- /usr/bin/mock       2007-06-14 19:42:22.000000000 +0200
+++ /tmp/mock   2007-06-17 23:30:46.000000000 +0200
@@ -606,6 +606,9 @@
             for line in child.fromchild:
                 w.write(line)
             w.close()
+
+            retval = child.wait()
+
             os._exit( (retval & 0xFF00) >> 8 )