This is a provisional draft.
As with most games, Final Fantasy IV has an assortment of glitches that can be used (and abused) to finish the game faster than without. This document attempts to establish a standard for which glitches are allowed in a "glitchless" run and to justify these decisions.
For various reasons, despite this being a glitchless category, some glitches are nonetheless allowed. Justification for these decisions will appear with each individual glitch. However, the general concept is that glitches that can be easily avoided are disallowed, while glitches that are automatic or unnecessarily cumbersome to avoid are allowed. Another important factor is the accessibility of the glitch. In other words, could a casual player likely discover or make use of it, even unknowingly? The glitchless run should not require the player to play in an odd, unnatural fashion simply to avoid using a glitch.
If a glitch or something that someone might consider a glitch at first glance is missing from this document, please feel free to contact me on Discord.
This glitch requires concerted effort to use and allows for memory corruption. Notably, it is the only glitch to be banned explicitly in a non-glitchless category, due to its ability to be used for a major skip.
This glitch allows the player to skip over event tiles on overworld maps. Its primary use is to enter Village Mist from the right side early in the game and obtain early access to several pieces of equipment. This requires significant effort to undertake and can be easily avoided.
The item duplication family of glitches can be used to either duplicate certain items or to obtain large amounts of money. While it is possible to start the glitch accidentally, it takes effort to make use of the glitch. An unsuccessful likely attempt to fix some portion of this glitch was made in Final Fantasy II (USA) (Rev 1).
This glitch allows the player to grind more quickly by using Life potions or spells on monsters as they die. The monsters are revived with 0 HP (as the HP Life restores is dependent on vitality, and monsters have none) and immediately die again, incrementing the monster type death count. It's unlikely this was intended, and requires effort to make use of (as it is unlikely you would use Life potions on monsters otherwise). It's possible using Life2 may be acceptable, though more discussion on that point may be necessary if anyone finds a practical use for it.
The Twin variant of the mimic glitch allows the player to put one of the twins in a mimic state wherein they can use items offensively that were not intended to be used in such a fashion. Using this glitch in a productive way requires significant effort.
This glitch allows the player to cause the MP of the target character to wrap around from zero to 65535, providing the character with virtually unlimited MP while in battle. (It does disable spell usage outside of battle, however.) This is very easy to avoid doing, and can be easily repaired with an Ether1 if accidentally triggered (though this is unlikely).
By using this glitch, the player can skip the second or additional forms of bosses by taking advantage of the fact that reflected spells do not trigger counterattacks, and that the monster phase changes are programmed as such. This glitch is more contentious as it can be easily (even unknowingly) taken advantage of in battle, using otherwise valid strategies (reflecting spells off one's own characters). However, it is traditionally disallowed in glitchless runs and provides the player a significant advantage in the Elements and Zeromus fights, and remains disallowed for the time being.
A casual player would no doubt quickly discover that Zeromus counters magical attacks and decide to begin reflecting spells to avoid these counters, and thus possibly unintentionally take advantage of this glitch.
By stealing a weapon from a monster (who has a weapon to steal), the player is potentially provided with a temporarily glitched weapon that, when equipped and used as an item, will cast an unblockable instant death spell. It is unknown if this has any practical use in a speedrun, but its use is nonetheless disallowed.
The warp glitch takes advantage of the fact that the Castle of Dwarves and Sealed cave crystal rooms share the crystal NPC. As a result, by casting Warp immediately following the Golbez battle, the player can obtain the Darkness crystal without traversing the Sealed Cave. The Sealed Cave is subsequently only visited to trigger a small cutscene. As this allows skipping an entire dungeon and is easily avoidable, it must be disallowed.
Once a character equips a back row compatible (long range) weapon, they are permanently able to attack without penalty from the back row (or against back row targets), even if they later switch to a non-long range weapon. Three characters (Rosa, Edward, and Rydia after she rejoins) join the party with a long range weapon and are automatically glitched. Therefore, unlimited use of their abilities should remain allowed.
The other characters are more nuanced. For most characters, it's not an issue as characters typically do not use long range weapons. However, most characters could conceivably equip a bow if desired. In addition, the Dwarf axe is used intentionally on Cecil for agility manipulation. One could simply require that from that point forward, any Cecil attacks must either be with a long range weapon or from the front row. This would not be particularly onerous. However, it does start to veer toward "overly cumbersome" territory. For the time being, it is advisable to adhere to this standard in runs.
In addition, this glitch appears to be allowed in Japanese glitchless runs and is easy to accidentally take advantage of in casual play. However, these same arguments can also be made to some extent to the Second Form Glitch, which is currently disallowed (though less cumbersome to avoid, though doing so does significantly restrict strategy).
As a side note, in the Japanese versions (except Easytype), Edward's harp is NOT a long-range weapon, and he is therefore not automatically glitched in those versions.
While Rydia must always have the requisite MP to select her Sylph spell, the MP is only deducted if she is in the middle slot. It seems like arguments can be made about this glitch in either direction, though it may be moot as it may not be particularly useful in a speedrun.
When swapping weapons in battle, if the character does not subsequently take an action, the stats of the previously equipped weapon will be retained. This is most useful in conjunction with Berserk or the Avenger. I'm personally inclined to allow this, as it's difficult to avoid when equipping the Avenger in battle and may be allowed in the Japanese category as well. In addition, casual players could easily take advantage of this unknowingly, especially with the Avenger.
It is possible to collect the treasure chest on Cave Magnes B4F by directly walking across the chasm. This is caused because FF4 tiles do not have any information on their boundaries. Rather, walking boundaries are determined by comparing two adjacent tiles to determine if they are compatible. In this case, the two tiles are compatible, so walking is allowed, even if it's somewhat visually jarring. In this respect, it's not really a glitch, but it is an example of either poor map design or an intentional choice to simulate a short jumpable chasm. However, it is no longer possible to collect in this way in the PSP version. Examination of the GBA version may be desirable.
In the Dark Elf battle, the dragon form is vulnerable to the Weak spell. This particular vulnerability was carried over through almost every 2-D version of the game. However, it was changed in the Pixel Remaster. However, the Pixel Remaster carries numerous other changes from the original version and is not a strong indicator of original intent. In any case, in this version, the monster lacks the so-called boss bit, and instead has several resistances to various status effects.
In the Elements battle, Valvalis is unintentionally weak to Lightning. This was fixed when the game was ported to later systems. However, this is easily discoverable in casual play, and as such, is determined to be a valid, if unintentional, game mechanic.
The Avenger, combined with abilities that temporarily remove the Berserk status (such as Zeromus's Blk.Hole spell) can cause a mimic effect similiar to that produced with the Twin variant of the mimic glitch. This glitch can be triggered accidentally, though using it in a useful fashion may require more effort. It is currently allowed, as it can be triggered accidentally. However, if someone devises a way to exploit it in an obviously beneficial way, this decision may need to be revisited.
Similarly to and in conjunction with run buffering described later, the player can exploit pausing the game to gain an advantage in battle, by executing only a single ATB tick at a time, allowing for precise run buffer timing. As with run buffering itself, this falls into the category of exploiting a game mechanic rather than a glitch.
After equipping a piece of equipment that offers elemental immunity, if the character subsequently removes that equipment, they will be permanently weak to the opposite elements. This may only be relevant to the Adamant armor (though I'd have to double check the equipment listing), and as such, is almost certainly irrelevant in any competitive run. It's dubious that someone would want to take advantage of this, in any case, as its only possible benefit would be killing one's own party faster, and there are sufficiently useful methods to do this already.
While messages are displayed at the top of the screen, the ATB is temporarily paused. While the player can exploit this behavior to gain an advantage, it's not really a glitch so much as a mechanic of the ATB system.
For most physical and magical attacks, statuses are assessed in terms of a rough priority hierarchy (determined by the order of the bits in the status bytes). By intentionally inflicting a character with one status, other statuses can be prevented. This behavior was explicitly written into the code, though their intention may not have been to provide an advantage to the player of this type. For the moment, this is ruled as an intentional (if somewhat perplexing) game mechanic, and allowed.
As part of his battle script, Zeromus adjusts his magic power before casting each spell. By triggering a counterattack after Zeromus has set the power for Big Bang (but before casting Big Bang), the magic power will be updated with a lower value for the Nuke counter. The following Big Bang will subsequently have reduced damage. Especially in the first round, this would be very likely to happen to a casual player, and disallowing it requires restricting magic use at arbitrary points of the battle.