Kirby's Return to Dream Land - Undead Magolor Glitch
To perform this glitch, you must have two or more players, at least two of which are Kirbys, and at least one of which has the Spark ability. At the end of the fight, when Magolor is charging in after losing his super-ability stamina, have Spark Kirby charged and positioned diagonally above and to the right a ways from Ultra Sword Kirby, then have him do the finishing attack. Before Magolor has a chance to break free or you can kill him, the Kirby with the Spark charged will deplete Magolor's health, causing the glitch.
After that, all players are unable to move or take damage, and the only way to leave the fight is to use the Start or HOME menu to restart. Thanks to my brother for helping me accidentally find out about this. I imagine it would work the same way with Magolor EX.
Additional audio from homestarrunner.com's "Strong Bad Email #174 - mini golf".