I've been playing endless mode, and twice I've had to kill myself because of the "spidernest" levels (I think that's what they're called - the ones with loads of saw blades, then stairs with walls each side with archers on top). What happens is that the Spidertron 6000 spawns, fires a load of spammy bombs at me, then despawns. The other enemies stay, and once they're all dead I don't hear the level complete sound. I've noticed other people missing an enemy (as in not killing one), but I checked the entire map 3 times and looked inside the walls, but didn't find any. Both times this has happened on level 11.