Roaming Pokémon Roar 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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Bulbapedia also has an article about Roaming Pokémon Roar glitch.

The Roaming Pokémon Roar glitch, sometimes shortened as the Roaming Pokémon glitch but not to be confused with the Roaming Pokémon individual values issue is a relatively minor glitch present within the Generation III Pokémon games, however through normal gameplay it only exists within Pokémon FireRed and LeafGreen because in those games, Raikou and Entei are the only Pokémon naturally capable of learning Roar by levelling up.

Usually when one of the Legendary Beasts flees, it should still exist within the game but would have fleed to a different route. If one of the Legendary Beasts uses Roar however, the game will not set this byte, suggesting that the player has run away or that he or she has defeated the Legendery Beast instead.

See also

1. Roaming Pokémon individual values issue