If used by a transformed Pokémon, Sketch will fail.
Generation II
Sketch allows the user to copy the opponent's last move permanently, therefore replacing Sketch. Sketch cannot be used to copy Selfdestruct, Explosion, Struggle, Metronome, or transform .
Generation III
The effects of Sketch continue into this generation; it can now be used to copy Selfdestruct and Explosion when used in a battle with a Pokémon with the Damp ability. It can be used to copy "calling" moves, such as Metronome, if the move fails to activate due to effects such as Sleep. This did not work in Generation II.
Generation IV
Same as Generation III. Sketch cannot be used to copy Chatter and Struggle.