There's a bug in cglib 2.1_3 that kills Hibernate in some instances, including the Opencast project in contrib:
http://opensource.atlassian.com/projects/hibernate/browse/HB-1520
We've found that moving to cglib 2.2 solves the problem, and we haven't encountered any issues in production (using a branch of the pre-K1 "db" module).
I'll attach a patch to this issue.
We should probably wait until after 2.6.0 is released before merging this to 1.0.x, to give a little more testing exposure.