As well as strafe-running and wall-running, the physics of the world of DooM contain some other rather strange bits and pieces. One is that the bounding box surrounding an object on the far side of a wall sometimes sticks out far enough that you can take the object from the other side of the wall. For example, in DooM II level 16, a shotgun from the megasphere room can be obtained from outside if you run diagonal against the right spot on the wall, where the texture shows a closed window. This COMPET-N LMP shows the trick in action.
A second strange feature is that linedef triggers that were activated as switches don't mind what height you are at relative to the switch (unlike ones triggered by walking and shooting.) Sometimes this allowed shortcuts that the level-designer didn't intend. A useful example of this occurs in level 11 of DooM II, where one can lower a barrier rather earlier than was intended and miss out getting one of the keys altogether. A second COMPET-N LMP illustrates this one.
Another bit of the game that seemed somewhat divorced from reality was the fact that you were battling hell-spawn from another dimension. Er, wait, no, that's not it. No, I was thinking about the BFG9000. This weapon worked approximately as follows. Turn in some direction. Fire a BFG missile off in that direction. Then, run to somewhere completely different and look in some other direction. The missile hits something, there is a brief pause, and then everything that would be in your current field of view if you were currently looking in the same direction as you were when you fired the thing gets hurt by the terrible power of the BFG9000. Hmmm. A little odd, but hey, it made the game interesting. Many a DeathMatch player was frazzled because they had not taken the time to read the BFG FAQ and understand the many intricacies of this most strange piece of hardware.