Using a brand new game of SIfixes with the 2016-12-03 build, I had a crash when going downstairs into Rotoluncia's dungeon. Shortly after unlocking the door the game crashed.
I have not been able to repeat this with either a reloaded save nor skipping to that area with a clean non-modded SI game, but it's a poor experience.
The actual crash has nothing to do with the illegal egg: that just happened to be there when the game crashed. We have been having these spurious crashes which generally happen after long sessions of play and a teleport. They can never be reproduced reliably, so we don't really know what is going on.
Is there a debug mode that would allow for Exult to create a memory dump when it crashes rather than just quitting the executable?
Not currently, but it is something worth looking into. But it is likely that it would not help in this case: it would point to were the crash happened, not to what corrupted the game state such that it crashed there. The only way to do that would be to do long play sessions under Valgrind (saving the log to a file), which would require disabling scalers to make Exult playable. All without the guarantee of actually encountering the crash: as I said, no one in fhe dev team ever encountered it, and we only get sporadic reports, meaning it happens very infrequently.