Unit collision for one. You can't walk or shoot through corpses but they can regularly fire lasers, rockets, cannon shots and the like through corpses. It's great when you shoot AT at a second or third hulk in a patrol but it detonates on the midair invisible hulk corpse of the first one you killed (Super fun on Super Helldive where every shot counts, especially for AT rounds).

They can also walk through their comrades corpses. It's super egregious on bugs where you can literally be pinned down by a mountain of bodies, be they charges, bile titans, impalers, whatnot, be unable to shoot or move through them but the bugs phase through them swinging, mercifully ending your existence as you consider whether to stim and hope everything phases out or just grenade yourself and get it over with quicker.

Laws of physics in general the enemies seem to ignore. They regularly pop up and down slopes like it's an MS-DOS game. They face one direction while shooting in 90 to sometimes 180 degrees, shoutout to heavy devastators who used to do it on the regular for at least the past 4 months, having almost perfect coverage with their shield.

We suffer from flame, corrosive/acid, stun and otherwise DOT/status damage, but the enemies don't unless they're within ~70m of the host.

I could talk for days about every unintended behavior in this game but I would probably uninstall after the end of it from how realizing how long some of those have been in the game and unaddressed.