When Justin was trapped in the quicksand, his left arm goes under and gets his sleeve all dirty. In another quick shot, his arm is above the dirt and the sleeve is clean. The next shot, his arm is submerged and the sleeve is dirty again.
During the Wizard Competition, Alex had created the first mud puddle for Justin to fall in and while he was digging around for his wand while she does the floating on water spell. When he finds his he shoots some magic at her. With the camera in the magic's point of view you can see a second mud puddle right next to the Big Magic (it's to the right of the table the ball of magic is on).
Justin changes sides on the wind surf before he runs into Alex.
When Justin and Alex are camping out for the night, Justin offers his shirt to her and partly takes it off, revealing his undershirt. In the next shot, however, Justin's shirt is completely on his upper body.
When Max takes the balloon and blows it up for Archie using magic he takes it into his right hand. But when he blows using magic it up it appears in his left hand. Then when they show an angle from behind Max the balloon is in his right hand again. Then when Max hands the balloon to Archie it is in his left hand.