New Super Mario Bros. Glitch
by jdaster12.8

In the 2-player Mario vs. Luigi mode in New Super Mario Bros., there is a spot where it is possible to go through a floor. Normally the thin, blue, blocky floors can be jumped through with a regular jump, but you have to go around them to go back below them.

In the 2-player castle stage, there is an area that is supposed to be reached only by Mini Mario containing 4 coins. However, with erratic jumping, ducking, and/or Ground-Pounds when the ceiling comes down, it is possible to go through the floor with any power-up, at least one out of every three tries.

This may be possible in locations in the 1-player game too, but so far I have not located similar areas other than a small overhanging ledge in the first tower level.