1. Look Mom, I can Walk Through Walls!
I was playing Medal of Honor: Frontline, and when I went to a specific spot(Never been able to do it more than once on each level) in Operation Repunzel, the game went wacky, blinked, and I was able to walk through walls, see down on the main platform that the designers used to build up the level, and could see Nazis that you can't even find in the game.
2. The Mystery of the Disappearing Fish-on-a-Board...
... in Medal of Honor: Frontline. I've found it in four levels so far. You can only see it for a split second, but I've seen it appear in Yard by Yard, Operation Repunzel, Arnhem Knights, and some other level. It looks like a sign that would be over a fish merchant shop.
3. The Wall that Men Could Walk Through...
In Clipping Their Wings on Medal of Honor: Frontline.
When you get near a hallway towards the giant tankard of molten lava, walk as far as you can until you near a hallway on your left, walk down in, get the bazooka ammo, and walk directly forward. There is a wall panel you can walk through and see the room with the giant tankard, another small room, and the beginning of the next level, Enemy Mine.