Bouncing off edges
There's a problem which is present in Hexen, and most likely on the other games too (but so far I only played Hexen), where you get bounced away from walls when you run into the edges of the polygons. That would kind of make sense if you ran directly into the wall, obviously you should get pushed away, however, you can be running almost (but not) paralel to the wall, if you get the edges of the faces, you'll get bounced away. Sometimes it makes for very frustating play, like the ledges when you're trying to access the Wastelands, if you try to run through the gaps and accidentally hit the wall, you're dead for good.
Just as of curiosity, it too happens on Hypostyle but in a kind of different way: the doors that close behind you will shoot you quite badly into the room (and you're jumped into it, so it's not just a simple forward force).