The Coin Case glitch

Most glitches bend one rule. This one hands you the pen. A single item, used just right, lets you write and run your own code inside Pokemon Gold and Silver.

Decoded from a Pokemon Crystal ROM · part of The Glitchhouse

When the game shows you the coins in your Coin Case, it prints a little line of text. In the English Gold and Silver, that text runs a command the game never meant to run, and the text engine leaps off its own list of commands into open memory. Whatever bytes are sitting there, your Pokemon's data, your box names, your stored items, get run as instructions. That is arbitrary code execution: not a bigger cheat, but the machine itself, briefly taking dictation from you.

Go deeper: arbitrary code execution (the Gen 1 sibling) | browse the Glitchhouse.