Continuity mistake: Alex's hair in season 3 changes back and forth between her shoulder length Bob and her long curly hair. The first instance of this is in the first episode with Mason in it. She had short hair in the previous episode, then it's long and curly in this episode, and then the following episode it's short again. This happens many times in season 3.
Continuity mistake: When Alex, Justin and Harper find Stevie's brother, he tells them Stevie left the hall of transfers before he could become a full wizard. In season 4 when Alex and Justin become full wizards, they do not go to the hall of tranfers but are given full powers by the headmaster.
Plot hole: When Justin and Juliet were trapped behind the glass and Juliet let herself be captured by the mummy, so she doesn't burn to ashes when the morning comes, why didn't Juliet just escape? The mummy gave her enough time to turn around and share a moment with Justin, she could've used that time to fight him off, break Justin out and escape. The mummy wasn't very strong and Alex would've shown up 20 seconds later anyway, there wasn't any real reason for Juliet to just let herself be taken away.
Continuity mistake: As Justin comes hysterically in to the restaurant from the lair, he runs over by the door to pull the shades down. A few seconds later when the camera is on Alex, you can see one of the shades is on the ground, However, after Alex says "you turned in your girlfriend to the Monster Hunter council" and Justin is shown, you can see that the shade has been put back in place.