|Please note that this glitch only exists in the Japanese versions of the game, or is otherwise a glitch from a Pokémon game which was only released in Japan.
The badge describer glitch is a Special menu Select glitch in Pokémon Red, Green, and Pokémon Blue (Japanese).
This glitch like the lift glitch involves the player swapping entries on a special menu, specifically the badge describer's list of badges in Cerulean City.
When the player swaps entries in this list, the game instead swaps pairs of memory addresses (as if the entry was an item and quantity pair). When the list pointer is affected (the lower byte of which can be manipulated by swapping entry 8), it allows for the player to change the location of the list to somewhere else in memory, leading to the corruption of unrelated memory addresses such as D036 (instant encounter).
Additionally having the badge describer talk about an invalid entry in the list can lead to glitch dialog and potentially arbitrary code execution.
A Super Glitch like corruption is also possible, but the cause is unknown.
Diagram representing pairs of items in the list. The text in red indicates which items control which two memory addresses when swapped as a pair
1. Prepare Master Ball in slot 1.
2. Deposit Pokémon 1, 0xFF, Blastoise in empty box (note that another pure Water-type may be possible).
3. Withdraw Pokémon 1 then Blastoise, should now be a 0xFF/Blastoise hybrid.
4. Send that Pokémon into battle, which should look like a blue Charizard or similar.
5. Run and then change to a box with Level 255 glitch Pokémon, withdraw it.
6. Enter badge man's house.
7. Swap option 2 with 8.
8. Swap option 1 with 2; CF18 to FF.
9. CFBF (any value?) followed by FF (after cancel after "green badge") to cf72 (pointer lower byte) - the FF is the user that entered battle.
9. Move up 39 times (cc36=2c) and press Select to place FF in CF72.
10. Swap option 1 with 2; CFFF to FF.
11. Find where D036 would be and make sure to place a non-freezing option at the top of the page.
12. Swap gure badge option to D036 (which has the 0x15 value from D000; Blastoise's type), then because D037 will also be set you should press A on the top item automatically.
13. If the item doesn't freeze the game Mew should appear at Level 255 in a glitch battle mode and you can throw a Master Ball to catch it if the Master Ball was in slot 1.
Arbitrary code execution
The Hyper Potion runs D333 in WRAM as if it was text code. By placing an 08 here the game will execute the bytes following it as code. Since D333 is part of the expanded items pack, you can swap items into place here beginning from item 73 (but they may go away if you change maps).