Case 1:
Mission group had missions for axis and allies. However, axis side route didn't have a flight.
When mission was loaded I picked the allied flight and client hung at briefing view. Had to kill process.
Case 2:
Same as above except there was no mission at all for axis. Loaded mission and selected allies. Client CTD at start of mission.
These are stupid designer errors, but given the ugly results at the client, seems like error cases the build mission validation should have caught.