During Finn and Rachel's duet, Rachel's necklace changes between being under or over the dress, depending on the angle.
During the song 'Smooth Criminal' Sebastian's hand movements change between shots and are repeated as he makes gestures in the background towards Santana.
Santana's coffee cup has nothing in it. It's only a prop. If there had been even a bit of coffee in it, there would be stains on the paper lining, and drops would have gone flying as she gestured with it in anger at Sebastian.
It's been established that Kurt takes French, yet he's sitting in Spanish class with Mr. Schue when Burt arrives to talk to him.
The note Rachel plays on the pitch pipe is not the starting note of "Ben" as sung by Kurt, Finn, and Rachel.