These guys were the only things to make me jump in a video game. I forget the level but there was a door and when it opened the room was black I took a step in and the lights banged on there is a Shambler right in front of me letting out a huge roar. LOL literally jumped back in my chair.
It was e1m6 "The Door to Chton" It also had a nasty moment when you had to pick up the golden key in the room full of lava while standing on a narrow ledge. When you grab the key, that son of a bitch spawned right behind you... So it is 99% that you end up fallen in lava or zzaped to death if doing this moment for the 1st time