GTS morphing glitch (Generation IV)
The GTS morphing glitch referred to instances in Pokémon Diamond and Pearl when a Pokémon found through the 'Seek Pokémon' option of the Global Trading System (now impossible to access in Generation IV due to Nintendo Wi-Fi Connection being discontinued from May 20th 2014) is displayed as the wrong Pokémon, with the picture of another Pokémon and often a nickname of that Pokémon. It does not change the original species name of that Pokémon however, resulting in many reports of players seeing Pokémon like a Mew with the nickname and picture of 'TORTERRA'.
The cause of this glitch is unknown.