Some obvious flaws are: impossible moves, over 510 EVs total, over 252 EVs in a single stat, IVs over 31, Egg Moves on a non-hatched Pokémon, unreleased abilities, impossible met/hatched locations... Some more complicated ones may be the Nature/IVs/PID/Shiny/TrainerID/SecretID relationship, which I'm not even sure still works that way (these values were all related in Gen 4). You may want a flame shield, since hacking Pokémon for online play, even if within the limits of the game, is heavily frowned upon here