I think at some point I am going to drop one from the most recent KODT into a dungeon. Basically there is a stone door in a corner, with a sign above saying "Only the strong may enter". Attempting to open the door will require str checks every round starting at 10 and going up by 2 each round all the way to 20. However, while pushing the door they are constantly depressing a spring behind it. If they fail the strength check, the spring releases and the door swings back slamming the PC into the adjacent wall, doing more damage for how far back they managed to push it.
The really evil thing is there isn't anything behind said door, and if they do manage to push it all the way back it releases and does the maximum damage possible. *evil grin*