The rule you broke is the following:
If you hacked any of your Pokemon for any reason make sure to mention it before your battle. You will be infracted if you are caught otherwise.
Keep in mind that if you are caught with illegal Pokemon you will be blacklisted. Pay particular attention to move combinations and Pokeballs. If you use a Pokemon that is not in a normal Pokeball and then use an egg move you are in violation of this rule as smart players can and will use the absence of a normal Pokeball to predict a lack of egg moves. (Exceptions include non-Colosseum starter Pokemon or fossils, who come in Poke Balls whether or not they have egg moves.) 3rd Generation tutored moves with 4th generation abilities (Counter/Superluck Absol comes to mind) or 4th gen exclusive Pokeballs (such as Dusk Ball) are also not legal hacks