I've discovered a couple of bugs.
1) Enemies in E2 can walk through walls, or rather doors. Some secrets in E2 connect to adjacent rooms, giving enemies a path through wich they can enter the room the player is. I never noticed this until the levels E2M10 and E2M11, though. Normally, they would sit where they are until the detected the player, which brings me to the second bug.
2) The Nail Gun upgrade, for the SMG, can shoot around doors and into the rooms behind them. Not sure how exactly this happens, but due to the bounce - and at certain angles - the player can shoot "around" doors. Let me be clear, the Nail Gun can't shoot through doors, only around them. Shooting directly at the door does not achieve the same effect. One has to shoot at the edges of the door, and at an angle in order to send rounds into room behind it.
3) The 2nd bug combined with the 1st bug, is how I discovered both. In E2M10 and E2M11, Some of the shots from the Nail Gun went through doors hiding secrets. Most of them connecting to adjacent rooms, the enemies in the other rooms would be wounded, and thus attracted to the player (me). Then the enemies would literally walk through the secret doors, and into the same room I was in, and begin attacking me - or as soon as they were within firing range.
Link to Drive with screenshots (because apparently they're too large to be attached directly): https://drive.google.com/open?id=1bzSpKnrSDs9--FPO3NI6AAJ19HOfoDf_
Update: It appears the Nail Gun shoots more easily through secret doors? Perhaps it's just stray shots finding that one corner/angle that it can pass around. Screenshots will confirm.