It is very well known that in Wolf3D, if you place a "deaf" enemy in one of the two corner tiles formed by two blocks joined only at the corners, the enemy will be able to see through the corner, and if the player walks into the other corner, the enemy will be alerted. However, it is seems that it is not very well known at all that if you have a Pac-Man ghost in one of the corners, the player will be hurt by the ghost if he/she walks into the other corner (because technically the player will be "brushing against" the ghost.)
This trick can be used to create a "red-hot wall" effect. If you create a wall where every accessible tile adjacent to the wall is a corner formed by two blocks joined only at the corners (note -- the wall has to be jagged) and place Pac-Man ghosts in the "opposite corners" (also note that the ghosts have to be moving, meaning that generally you should do it by having an "area" behind the wall where every single tile is occupied by a ghost and floor code 0000 ("Invalid") is assigned to this inaccessible area,) the wall will be "red-hot", meaning that if the player stays long enough in any one tile, he/she will start taking damage very rapidly. You should also make every "ghost area" behind the wall as small as you can (often only 2 or 3 tiles is necessary) and try to (although you often can't) have a separate "ghost area" for every single "red-hot tile" to prevent the ghosts from "clumping together" (remember that ghosts are "transparent".)
The general thing to do with this trick is to create a "red-hot corridor" -- where if the player stays long enough in any one tile, he/she will start taking damage very rapidly (for good measure, you can put enemies in the corridor as well.) Another thing is to place a single red-hot wall in an inconvenient location (note that in this case, it can be more interesting if the "red-hot walls" are of a different texture.) You can also have "red-hot doors", by having all accessible tiles adjacent to a door "red-hot", "red-hot objects", by having all accessible tiles adjacent to an object "red-hot" (or, in the case of penetrable objects like piles of bones, the tile that the object is on, which can even be used to create "poisonous blood" or "poisonous water".)