Use a P-Wing in Bowser's Castle and when you reach his chamber, fly up the wall on the left. When you reach the top, keep flying left. If you drop to the other side of the wall, you'll fight Bowser in that room instead, but that's not the real glitch. Instead of dropping down, run back the way you came and fight Bowser in the first room you entered. He'll have no fireballs, even though he's still trying to breathe them out.
Taken with permission from Flying Omelette.