The undead glitch is a glitch in Super Smash Bros. Brawl and Super Smash Bros. 4 that allows Giga Bowser, Wario-Man, Giga Mac, Mega Charizard X, or Mega Lucario to survive after being defeated in Stamina Mode.
When this occurs, if Giga Bowser, Giga Mac, Mega Charizard X, or Mega Lucario's HP is depleted, their stamina gets reduced to 0 HP (zero hit points). Then they are able to continue to fight; despite having "0 HP" displayed. Being touched by any hitbox even if it deals no damage (a Banana Peel, F.L.U.D.D. from Mario, or Water Gun from Squirtle) or hit by a status move (such as being put asleep or using recovery items which normally recover HP like Food) gets the character KO'd. A 0 HP Giga Bowser, Giga Mac, Mega Charizard X, or Mega Lucario can freely get another Smash Ball to trigger the glitch again.
With the exception of Wario-Man in Brawl, it's possible that an undead character (like Giga Bowser, Giga Mac, Mega Charizard X, or Mega Lucario) can win a stamina match if they remain untouched (with 0 HP). If they defeat all of their opponents as described above, they (with zero hit points) can win a match. Wario-Man, however, is the exception (Brawl only). Upon triggering the glitch with Wario-Man in Brawl, he loses his damage meter, and is therefore counted as having been KO'd. This also means that if the match is 1 on 1 then the match ends when Wario-Man get's KO'd. This is not the case when doing a 3-4 player VS. If Wario-Man (in Brawl) defeats his opponents, the winner becomes the last opponent who was defeated by Wario-Man's attack. However, in Smash 4, Wario-Man behaves like all characters.
Characters that can induce the glitch