Game:

Pokémon Crystal (2000)

Duration: 0:53

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New description as of how I wrote it for The Cutting Room Floor (still needs revising):

This bug only occurs in a battle without any Pokémon. The core bug was also carried forward to Pokémon Crystal. The only intended example is Professor Oak finding the Level 5 starter Pikachu and catching it, unique to Pokémon Yellow. In order to determine the battle transition, the game checks:

Is the battle not a Trainer battle or from a dungeon map?
After concluding that it isn't, the game checks the player's lead (non-fainted) Pokémon's level in comparison with the opposing Pokémon's level (5) in order to decide a "single circle" or "double circle" transition. A double circle is chosen if the Pokémon's level is +3 of the enemy's level, else it will be a single circle.

Due to a bug, the game will check beyond slot 6 of the party (with the player having no Pokémon at the time), while looking for the first non-fainted Pokémon. The first Pokémon beyond slot 6 will be Pokémon 12 if the rival name is greater than 5 characters long ($D34F; rival name character 6), where at least the end terminator (0x50) byte of the rival name is read as the HP (higher byte) for a Pokémon beyond slot 6. A non-00 value will mean it has remaining HP, so is not fainted. The level of Pokémon 12 is determined by the map's north connection byte, which will also be a value of 12 (0x0C; meaning the north connection is map 0x0C; Route 1. 12+3 (15) is greater than the opposing Level 5 Pikachu).

Therefore, in a nutshell; a rival name (internally ignoring terminators) greater than 5 characters long will ensure under a double circle in normal play (values 01 and above, because the map connection value of 12 is greater than the Level 5 Pikachu).

A Rival name can meet this crtieria if:

A pre-set name is chosen. Note pre-set names have hidden characters beyond the terminator.
A custom name was chosen for the rival, consisting of at least 5 characters (excluding the terminator, which might count as the sixth).
On the input screen, the player entered at least 5 characters, but erased them with B, leaving stray 0x50 bytes in place after confirming the name.

For this reason, speedrunners who know about this bug may consider it as a reason (depending on the category) to choose a pre-set name/longer rival name/rival name with characters erased with B; because the 'double circle' battle transition is ~30 frame faster.

(Source: Crystal_ (documentation for Pokémon Crystal), TiKevin83 et al https://tasvideos.org/6119S)

Old description:

Notes: As Crystal_ has documented, a transition before battle is influenced in specific ways    • Pokemon Red/Blue - Curiosities of bat...  

Namely, in this case the battle transition can be one of the below two (as it is not a Trainer battle or dungeon map).

Single circle (wild battle, no dungeon map, player's level +3 is less than or equal to enemy level) (if rival's name isn't less than 6 characters)
Double circle (wild battle, no dungeon map, player's level +3 is greater than enemy level) (if rival's name isn't greater than 6 characters)

A variation occurs when the Rival's name is greater than 5 characters (including hidden characters on pre-set names). This is due to the 'party Pokémon 12' beyond slot 6, which has its current HP byte 1 from Red/Blue $D350(?)/Yellow $D34F (counting as an active Pokémon with over 0 HP but actually rival name slot 6) and Red/Blue $D370(?)/Yellow $D36F (counting as Pokémon 12's level (the second level address in the structure), but actually the map's connection byte).
Source: https://github.com/pret/pokeyellow/bl...

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