We hit a situation whereby a bug in the domain code was causing a JDOUserException to be thrown (missing a mandatory-field), but then throwing an org.apache.isis.applib.ApplicationException.
This exception is only intended for non-fatal exceptions. Isis therefore attempted to commit the transaction subsequently only to find it had already been aborted.
ObjectActionImpl should try to identify this programming error, and "do the right thing", which is probably to throw the ApplicationException's underlying cause (else a runtime exception if there is none).
NB: this ticket should also include adding some documentation to the website, since this is a pretty subtle issue.