Plot holes: Joe twice declares that he lives by the rules, leave no traces. Yet throughout the movie, he handles his guns and ammo with bare hands--leaving fingerprints and DNA all over them.
Continuity: Joe shoots two men while his girlfriend isn't looking at him. Blood splatters all over his girlfriends back. The next shot of her back has hardly any blood at all.
Continuity: During the final shootout, the gun Joe is using switches between a Glock and a Sig Saur between shots.