When Artie is singing and dancing in the hall during the song "Dancing With Myself" his support wheels on the back of his wheelchair vanish and reappear between shots.
When Artie is singing "Dancing by Myself" the trip leavers keep disappearing and reappearing.
Near the end of the "Defying Gravity" scene, Kurt visibly mouths the words to the "I'd sooner buy Defying Gravity" part wrong.
In the scene where Kurt is rehearsing alone, attempting to sing a "high F", the notes he sings are not the same as those he plays on the piano. By the time he presses the "F" key on the piano, his voice is actually one and a half to two whole steps above the "F".