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Train station

Train platform, the first area of Final Fantasy VII.

The Quantum Potion bug is a programming mistake in the first screen of Final Fantasy VII. The train platform for Sector 1 Reactor has two unconscious Shinra guards from which the player can retrieve items by checking their bodies. However, due to a mistake in copying the code, the same items can be retrieved from either guard: retrieving a Potion from one guard removes it from the body of the other guard, as if the Potion existed in a quantum superposition of states, hence the name Quantum Potion bug.

Quantum PotionsEdit

When an item is collected from a field location or chest, a variable is set to record that the item has been collected to prevent the player from picking up the same item repeatedly. A separate big relating to this is the Infinite Elixir glitch the player can perform at Great Glacier. Both guards at the train station have two Potions, but they share the internal variable to keep track of if the Potion(s) have been collected. Therefore any of the following combinations is possible, but it is not possible to get more than two Potions:

  • Get two Potions from the lower guard's body
  • Get two Potions from the upper guard's body
  • Get one Potion from each guard (lower then upper)
  • Get one Potion from each guard (upper then lower)

PseudocodeEdit

The code for both guards when inspecting the body is as follows:

If Var[15][32] Bit 0 is OFF
 {Display "Received Potion!"
  Add 1 potion to inventory
  Set Var[15][32] Bit 0 to ON}
If Var[15][32] Bit 1 is OFF
 {Display "Received Potion!"
  Add 1 potion to inventory
  Set Var[15][32] Bit 1 to ON}

Third guardEdit

There is a third guard in the game code for this location that shares the same variables as the first two guards, and would thus also be subject to this bug. However, the third guard has the field model hidden and the ability for Cloud to interact with it disabled, so the guard is invisible and cannot be spoken to to retrieve the Potion(s).

OriginsEdit

It is likely that this bug originated from an incomplete copy-and-paste process of duplicating the first guard. If the original programmer had remembered to change the variables used for tracking items, then this bug would not have happened. The existence of a third, invisible guard adds weight to this theory as it is another leftover copy.

The glitch remains in all versions, including PC re-release, PlayStation 4, iOS, and Android.

TriviaEdit

  • The same bug happens in Nibelheim with a different set of items. The black cloaks around town yield various stat-boosting sources when spoken to. The one outside the Shinra Manor gives a Mind Source, but the text box has an error, saying "Mind Plus". Like in the quantum Potion bug, the same variable is used for obtaining this Mind Source as for the Luck Source obtained from the black cloak on the bottom floor of the inn; when the player picks up either one, the other registers as already being taken, so it is impossible to pick up both. Because players are likely to visit the inn before the mansion (as the party arrives from that side of the area), many likely never obtain the Mind Source.