Miss Holliday's umbrella switches between black and yellow several times during the closing number.
When Mr. Shue is hallucinating all of the Glee club members as toddlers, two little girls in Cheerio uniforms (meant to be Brittany and Quinn) are sitting in the front row between Artie and Sam. However, when Mr. Shue's back was turned to the group, only one of them was sitting in the front row.
During the "Singing in the Rain" scene a 3-second clip is playing in reverse.