The Doom bug is a bug in the Japanese SNES version of Final Fantasy IV. If a character is under the Doom status, and the player holds 
throughout the countdown, once the timer hits 0, the timer will never go further, thus the character under Doom will never die. This is especially helpful against the Plague Horror boss, as it never deals direct damage and thus, the player can defeat it without worrying about dying from Doom. This bug was fixed in the English release.
The Doom bug is a bug in the Japanese SNES version of Final Fantasy IV. If a character is under the Doom status, and the player holds throughout the countdown, once the timer hits 0, the timer will never go further, thus the character under Doom will never die. This is especially helpful against the Plague Horror boss, as it never deals direct damage and thus, the player can defeat it without worrying about dying from Doom. This bug was fixed in the English release.