The Sketch glitch was a glitch in the Nintendo Space World 1997 demos of Pokémon Gold and Silver, which was documented over 20 years later in 2018, after the demo ROMs were leaked online.
In the final versions of Pokémon Gold, Silver and Crystal, Sketch functions correctly and copies the last move used by the opponent.
In the trial demo, it would instead replace the opponent's last move with Sketch permanently. The remaining PP of the opponent's last move is unchanged even if it is over the maximum PP of the move (1).
Sketch is the signature move of Smeargle (Japanese: ドーブル, Doble), known as Painter (Japanese: ペインター) in the trial demo.