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Sometimes this includes Pokémon with moves at an earlier level than possible with Pokémon owned by the player, moves that weren't available for the Pokémon until later games and even moves that Pokémon have never been able to learn.
Probably three of the most well known examples include Lance's Dragonite in Generation I knowing Barrier (which was illegal until Generation VI), Lance's Dragonite in Generation II knowing Rock Slide (which was illegal until Generation III), and a Lanturn in Pokémon Emerald that knows Earthquake.
Two more recent examples of these errors include two errors in Pokémon Sun and Moon that were later fixed in the v1.1 patch, including a Kommo-o in the Battle Tree knowing Shell Smash (replaced with Draco Meteor) and a Shuckle in the Battle Royal knowing Stockpile (replaced with Double Team).
Bulbapedia covers this subject in more depth.