Focus Punch glitch

Miscellaneous glitches of Pokémon Ruby and Sapphire, Pokémon FireRed and LeafGreen and Pokémon Emerald

Exclusive to Ruby and Sapphire:
256x item selling price glitch | Amazing Trainers | Berry glitch | Draining hitpoints animation glitch | Evolution animation glitch | Sticky Hold glitch | Mossdeep City Skitty glitch | Truck glitch | Nightmare glitch | Mossdeep City Skitty glitch
Acro Bike instability glitches (Japanese only) | Dark red selector glitch (International only) | Excessive doll arrangement glitch (Japanese only) | Mail and Trick glitches (Japanese only) | Surf on land (Japanese only)

Exclusive to FireRed and LeafGreen:
Celadon City origin glitch | Pokédex species glitch | Roaming Pokémon Roar glitch

Exclusive to Emerald:
Battle Pike poison knockout glitch | Black curtains glitch | Mart worker glitch | Mover Vigoroth cry glitch (Japanese only)

In various or all entries:
Azurill gender glitch | Boss HP recruit glitch (Mystery Dungeon) | Decamark-specific glitches | Dewford Gym footprint glitch | Evolution move-learning glitch | Focus Punch glitch | Roaming Pokémon IV glitch | Sky Pillar glitch | Thunderbolt glitch | VBlank RNG interrupt glitch

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The term Focus Punch glitch refers to a minor oversight regarding the move Focus Punch when it is used in double battles in the third generation of Pokémon games. The glitch was fixed in the Generation IV games.

When exploited, the game will display the message that the relevant Pokémon is 'tightening its focus', although instead of usually performing the move Focus Punch or failing, when the glitch is exploited another action can be performed instead of the attack after the message is shown, such as the player switching the Pokémon.

Requirements

1. Access to a double battle.
2. A Pokémon knowing the move Focus Punch.
3. Preferably, an item may be used instead of switching the first Pokémon.

Method

1. Enter a double battle, with a Pokémon knowing Focus Punch as the first Pokémon.
2. With the first Pokémon, choose to use the move Focus Punch.
3. On the second Pokémon's turn, press the B button to return to the first Pokémon's turn.
4. Either switch the first Pokémon or use an item and then choose any action for the second Pokémon.

If the player had chosen to switch the first Pokémon, it will tighten its focus but immediately be switched out after the message is displayed. If the player used an item, the item is usable as normal and the Pokémon would attempt to tighten its focus, but not perform the move, even if another Pokémon did not attack it.

Explanation

When a player's Pokémon uses the move Focus Punch a flag is checked so that at the end of the player's turns his or her Pokémon will attempt to 'tighten its focus' with high priority. A simple oversight for double battles however, was that the same address remains unchanged when the player presses the B button to return to the first Pokémon's turn, so another action could be chosen with the flag still checked. By using a move other than Focus Punch the flag is unchecked for the relevant Pokémon, although another action such as switching a Pokémon or using an item will keep the flag in its previous state, hence it is possible for that Pokémon to have tightened its focus but not necessarily use Focus Punch at the end of the turn.