When the guys sing "Beth", you see Finn sitting up front singing, then see him behind Quinn, then upfront still singing, then no longer behind Quinn.
When the Glee Club is performing "Bad Romance", one bubble from Tina's clothes falls from it and starts rolling on the floor. In the next shot, it disappears.
The toy frog that falls off Rachel's dress disappears without anyone picking it up from the floor.
Both Puck and Artie are playing six-string guitars in "Shout It Out Loud," yet there's a bass part in the music mix.