Stiles' shirt changes from white with black trim to a gray shirt with palm trees on the pocket in the middle of the scene where Stiles and Lydia are in the Chemistry Lab.
When Scott picks up the second piece of sushi the shot flashes to the parents then when it's back to Scott the sushi is no longer in his chopsticks - and not enough time has passed for him to eat it.
The gash on Scott's head keeps changing size and position.
When Stiles and Lydia are looking around the chemistry classroom for clues, there is a quick change of shot where Stiles is wearing a different t-shirt to the one he wears for the whole episode. After this shot, he resumes to wearing the previous shirt.
The staple gun that Burrows uses to close his wound is the wrong type. This type will just push the staple in but wont close it. Movements he makes will just push the staple free.