What I had in mind was something like a NPE because one of your process variables is not initialized, for example a programming error in your form validation.
I don't think low level exceptions like this should be handled in a higher level context like the Process Engine, you could probably initialize a transition into an "error" state but this would violate the "Crash, Don't Trash" principle.
Having said that, I also think it probably makes sense in some cases to have an exception handling in your process definition. As far as I know this is implemented in jBPM3, see: http://docs.jboss.org/jbpm/v3/userguide/jpdl.html#exceptionhandler.element, unfortunately jBoss stopped development on jBPM4 before this was implemented in jBPM4.