In Level 2, after the stage with the chandeliers, jump over the fire and you will get to a part where there are two stones and two boxes. Smash the stones with Wakko and pull back one box with Yakko. Now, make sure that the second box is where you it was when you smashed it. If you walk all the way to the right and then back, the other box will disappear! It doesn't hurt the gameplay since you can still do whatever the game asks for with one box.
To get the box back, just walk all the way left and back.