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.
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.
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).
When Alex says "You want magic? There's your magic" as she mocks Justin, Max has a soda bottle in his hand. When he goes and volunteers to be in Archie's 'Magic Show', there is suddenly a straw in his soda bottle.
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.
When Justin reaches his arm out to stop Alex the cliff has no foliage at all from the below view. Yet when you look from the overhead view you can see plenty of foliage up to on top of the cliff hanging out over the edge. Also when you see Justin and Alex from the top view they are multiple feet from the edge of the cliff.
When Alex and Justin are taking wind surfing lessons you see there parents whose shadows are facing towards the ocean. Then Right before Alex is crashed into by Justin if you freeze the frame you can see that Alex's shadow is parallel with the ocean. It is obvious the the parents were filmed at a separate time.