In the episode with Frank Grimes, Bart buys an old factory at an auction. In one scene, he and Milhouse are playing "Wacky Shaft" on top of an old wooden staircase. Milhouse then sees three warning signs and alerts Bart. So Bart proceeds to throw two of those signs out the window, but when we see the staircase again, all three signs are gone! Where'd the third sign go?