If used by a transformed Pokémon, Sketch will fail. If used in a link battle, any move learned via Sketch will be forgotten at the end of the battle.
Sketch allows the user to copy the opponent's last move permanently, therefore replacing Sketch. Sketch cannot be used to copy Selfdestruct, Explosion, Struggle, Transform, or Metronome.
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. Struggle is the only move that cannot be sketched.
Generation IV and onwards
Same as Generation III, except that Sketch cannot be used to copy Chatter and Struggle.