Continuity mistake: In the "Wizards vs. Werewolves" special, just as the Russos learn that Alex and Mason have gone to Transylvania, Max is playing on his Nintendo DS. He then flips it shut. The shot switches to Justin, Jerry and Teresa. They talk and the shot switches Back to Max, and you see him flip the Nintedo DS shut again.
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: When Alex's mother is telling Alex that she can help Grandma with the chickens during the summer, she tosses Alex's hair back and we can only see Alex's back with the hair. When the camera pans immediately after, it shows Alex's face and her hair back where it was before her mother moved it.
Continuity mistake: After Justin and Juliet come back from the movie, they sit down in front of the sub station. When Justin and Juliet talk, look at Juliet's left shoulder; her sweater goes from being on her shoulder to slightly off her shoulder every time the camera pans back and forth on them.
Continuity mistake: They learn the miriana animata (make an inanimate object come alive/walk/talk) but in episode 10 they already learned this one. Alex made Justin's pimple come to life (EP 10), so why was she so confused in EP 19 about how if it gets feelings, she can't make Manny the mannequin inanimate again? Her, Max and Justin (to an extent) made it as though they never learned this one or had troubles with this spell.
Deliberate mistake: Justin tells his dad he will write the idea on the white board (the theme in the restaurant/add more of an experience) yet he just scribbles, and in the scenes where you see a bit behind him, the whiteboard has scribbles instead of words where he was supposed to write.
Continuity mistake: In episode 25 of season 4, Harper mentions still being in school and even goes to school during the 50's episode to keep her "perfect attendance." But a few episodes earlier, her and Alex both graduate, and even have an apartment. The episodes seem out of order.
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.