'ERROR!' is an unused option string that can be forced on an options box for the party Pokémon screen. Its purpose if anything is undocumented.
It follows on from 'MAIL' (hex:14); the last valid option. Field moves have index numbers between hex:01-0E, so "ERROR!" probably wasn't supposed to be one.
Enter the GameShark code 0115D6D0 to replace the third party menu option with "ERROR!". All ERROR! does with this code is function like cancel.