I've gotten this crash a few times, and I'm pretty sure that it happens when the last monster in a wave is a cacodemon who gets killed by the explosion of it's own projectile. I haven't tried to seriously test it, but for me, the crash always seems to happen when that situation occurs, and never at any other time- and it makes sense that it might confuse the game, since the player didn't actually kill the last monster.
Haven't posted in a while, but I just wanted to say I'm really enjoying the new version. It seems like it's even harder than the last one, though that may be my perfectionist streak coming out. I always have to try to become the arena champion, rare as it is that I actually manage to do it...