From Glitch City Laboratories
Two error messages were implemented into Pokémon Gold, Pokémon Silver and Pokémon Crystal to prevent players from trading invalid Pokémon. These messages can appear as either "Your friend's (Pokémon) appears to be abnormal!" or less commonly "Sorry, but your POKéMON appears to be abnormal. We can't accept it."
Their shared purpose is to prevent players from trading Pokémon with invalid data across games, particularly those with use an invalid species identifier (i.e. "glitch Pokémon") after Nintendo's acknowledgement of the Old Man Trick. The message can also appear when one player attempts to transfer for example, a Rhydon knowing the move Fly.
The message "Sorry, but your POKéMON appears to be abnormal" prevents the user from sending an illegitimate Pokémon on to another game, for example when one attempts to trade a bad clone across other Generation II games. Similarly, the message "Your friend's (Pokémon) appears to be abnormal" informs the player that the other player has an invalid Pokémon, for example when one attempts to trade a glitch Pokémon from Generation I onto Generation II via the time machine. In this case any Generation I glitch Pokémon would appear as a Generation II Pokémon with the equivalent identifier minus 39 (decimal). For example, a h poké would appear as a Quilava but would be untradable, as this 'Quilava' would be blocked by the error trap.)