The W-Item-Glitch is a very evil glitch that essentially breaks the whole game. If you decide to abuse it, the game is pretty much over immediatly.

W-Item is a Command Materia which you can only get at the end of CD2 or during CD3. It allows you to use two items in one turn. Unfortunately, due to a glitch, you can use it to duplicate any item - that is usable in battle - infinitely. So yeah, really game-breaking.