Something is wrong :)

To analyze and eventually solve the problem however, we need a guide to reproduce the problem quickly. While testing, we may need to try the same situation a few dozen times, and a guide like "play corsixth for a few weeks" doesn't quite work then.

Do you have a way to get this problem again in say 1 minute? Usually it starts with loading a game, then letting it run for 10-30 seconds doing nothing, or doing some specific instructions, leading to the crash.
If you have, please upload the savegame, and tell us how to cause it to crash again.

The isOperating check in OperatingTheatreRoom:commandEnteringStaff is doing a check on staff_member_set for any staff member with a multi_use_object action.

It then assumes that 'surgeon1' derived from next(staff_member_set), is the one actually performing the operation. Though admittedly its a little bit harder to find when that won't occur, but could spontaneously occur during depersistence I guess or when the insert into staff_member_set is just right.

However I was able to interrupt an operation, and rotating doctors in quickly into the one operating theatre and replicate without my getStaffMember change. In trying to replicate it other attempts I had 4 gowned and operating surgeons at one point and I was able to drop a gowned surgeon outside in the corridor. But the circumstances of doing that would be outside of 'normal' play.

So far unsuccessful with the getStaffMember function changes guarding the commandEnteringStaff call in replicating the issue. isOperating could just return the staff member instead and the assert in this regard becomes redundant (but believe its more for the other next(self.staff_member_set) references where this is also handy.