There is a very large fish tank in Sean Fretthold's bedroom when Finn and Rachel go to visit him. When Rachel returns to visit Sean by herself the fish tank is gone.
During Puck's number the brim of his hat goes up and down.
Puck introduces his song, "The Lady is a Tramp", as one of Sammy Davis, Jr.'s biggest hits. The song was a never charted hit for him.
After performing an atypical song for his personality, Kurt says, "Well, I'm sorry if I didn't live up to your expectations... I'm not a box. There are more than four sides to me." A square has four sides. A box which is generally a cube or cuboid (a rectangular prism or 3D rectangle) has six sides. Since the conversation continued normally without so much as a pause at what he said to indicate this sounded odd, it must have been an error.
Quinn says that Puck's mother won't let her eat bacon, but Puck had previously mentioned eating sweet and sour pork as part of a family tradition while they watch "Schindler's List" together.
Burt says "Nice to meet you" to Brittany, but he met her previously, when she and Tina were dancing "Single Ladies" with Kurt in his sequined unitard.