The Regen bug is an exploitable glitch in the PlayStation version of Final Fantasy VII that can allow players to restore characters' health unimpeded by enemy attacks. It works because most abilities in the game must be loaded from the disc as they are used, and if the disc is not spinning in the tray to allow the console to read from it, the data cannot be loaded, causing the battle to hang indefinitely. However, the game will still allow "time" to pass in the battle, meaning that Regen continues to take effect. This only works in Active mode, as Wait mode pauses all time-based calculations, including Regen.
The player must first cast Regen on any or all of the party members and then open the disc tray before a unit takes action. The game does not run until the disc tray is closed, but the characters will continue to gain health. Effectively, one can cast Regen, take an attack, then open the disc tray to regain the health.
Poison does not behave the same way as Regen, and will not work with this exploit. Regen is a constant rate over time, whereas Poison is chunks of damage at intervals.
Due to the nature of the exploit, it only works when playing a physical copy on an original PlayStation console. It does not work in the PC versions or when played on the PlayStation 2. It also does not work on the PlayStation 3, as the game will shut down if the disc is ejected.