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.
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.
Wizards vs. Vampires on Waverly Place - S2-E26
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.
Rock Around the Clock - S4-E25
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.
Suggested correction: Yes, episode 25 "Rock Around the Clock" was the 14th episode produced (production code 414) and occurred before they graduated the 17th episode "Wizards vs. Asteroid" (production code 418). This isn't a TV mistake since it was produced in order.