A glitch is an oversight in programming of a video game and how it may act in some circumstances. They alter gameplay by causing events or actions to occur that were not intended by the game's designers. The main reason for a glitch existing is that a specific situation resulting in unusual behavior was never encountered during a game's testing, so the developers were unaware there was something that needed to be fixed. For example, the regenerating terrain glitch exists because the situation of "terrain appears around character using Final Smash" was not fully tested by the developers. The more complex a game is, the more likely glitches are to exist, simply due to the increased difficulty in testing every possible situation. Glitches are different from exploits, which involve multiple gameplay elements yielding unintended results.
The effects that glitches cause can vary. Glitches can be harmful, with such side effects as freezing the game, causing a level to be unplayable, or KOing a player for no apparent reason. Not all glitches, however, have negative side effects; some can allow the player to continue playing the game as usual, such as the edge running glitch, while others can merely be done for fun, such as the Name Entry glitch and all of its variations. Some can even be considered helpful, such as the regenerating terrain glitch on R.O.B.. Stages or moves that mostly work as intended but have strange characteristics are usually called “jank” (i.e. Mario’s dash attack in Smash 4 stage spikes opponents on the ledge, or Pokémon Stadium’s odd collision detection during the Fire transformation).
Tournaments lack rulings on or against most glitches as they usually cannot be combined with a player's ordinary tactics. A majority of glitches have prerequisites that are unattainable in a tournament setting, such as items, Final Smashes, more than two players, or other, similar limitations; furthermore, many glitches simply have useless effects that would be of little benefit to players. Glitches with practical effects, such as the Teleport in Smash 64 and Yoyo glitch in Melee, are generally allowed to be used, as they are perceived to add to the afflicted characters' metagames. Glitches with only cosmetic effects, such as Melee's stock glitch, are considered unobtrusive and thus are also usually allowed in competitive play. However, glitches that break the game or otherwise cause matches to become unplayable for any party, such as the Freeze glitch in Melee and the Infinite Dimensional Cape in Brawl, are banned in tournaments. The above applies to world records and speedruns as well: while most glitches are permitted, some with game breaking potential, such as the pause glitch in the original Super Smash Bros., are discouraged and records made using them are not recognized by formal leaderboards. Other times, a category may be split between not allowing certain glitches (or none at all) and permitting them.
If discovered, glitches can potentially be patched out in later revisions of the game, as has happened with all Smash titles barring the original and Brawl. In Melee, glitches had the potential to be removed in future revisions and rereleases, such as the removal of the Shadow Glitch in all versions of Melee after 1.0. Smash 4 and Ultimate improved on this formula, by allowing these games to be potentially patched after release via downloads from the Internet. Version 1.0.4 for the 3DS version, for instance, fixed a glitch where characters could grow to immense sizes in Multi-Man Smash via the use of Yoshi's Egg Lay.