Continuity mistake: After Mrs. Russo puts Justin and Max in the lair, she is wondering if she should put the big lock on the door or not. She decides not to and puts the lock down on the counter. Two shots later you see the lock is back on the door, but unlocked. You never see Mrs. Russo put the lock on the door.
Continuity mistake: In the episode "First Kiss," Justin has a girlfriend, Miranda. But in this episode Justin starts dating this new girl from his class, who's actually the same Miranda. This doesn't make any sense. How is she a new student and how could they just meet if they were dating eight episodes ago?