I fell off the "bridge" and fell into the toxic floor multiple times. Besides that, I was like Lüt: I couldn't find my way out of the blue key room after getting the blue key.
I kept on wondering why everytime I entered the dark room with all the demons and imps the game would restart back to the title screen after app. three seconds. :P
E2M2 - Refinery:
I could barely make it past the room with the berserk pack
E2M4 - Halls of the Damned:
"Wow, this is the spookiest map I played yet..."
...I said with 45% health...maybe 25%...slowly and cautiously creeping through the long STONE1 corridor expecting something BIG to show up, but to only find three more passageways at the other end.
/me begins to hum Sinister...