Puck receives quite a few punches to the face from Finn after finding out the baby is not his. In the next scene (Puck talking to Quinn in the hallway), Puck's face appears to be unmarked.
Shortly after Rachel moves to the stage while performing "Don't Rain On My Parade", a bandage can be seen on the side of her right leg, just below the knee. In the very next shot a few seconds later, the bandage is gone.
Toward the end of Rachel's solo at sectionals, the rest of the group comes on stage. In the close-up, you can clearly see Finn and Puck cross paths and stop behind her. Then, in the wide shot, the boys are still walking by Rachel at the front of the stage.
When buying pretzels a part of the man's pretzel falls on the floor, in the next shot the pretzel appears whole again apart from the small bit he's holding.
The club asks for Will's help in setting the set list for sectionals and Will says he can't help them. However, the set list is already in place, as Sue has leaked it to the other two teams. This is not an error because the set list was given to Sue by Will at the end of Hairography. Clearly it has been changed since then, since "And I'm Telling You" was added. Later Brittany admits that she gave Sue the set list, which was probably the updated version with Mercedes' ballad.
When she is accused by Will Schuester of leaking the set list to the other schools, Sue Sylvester says that the accusation is a "libelous statement". Libel is written, slander is spoken; the correct term would be that it is a "slanderous statement".
When Emma confronts the directors of the rival glee clubs, she wonders if the deaf students will sing "Don't Start Believing". The correct title is "Don't Stop Believing".
After Will leaves rehearsal and Emma tells Rachel to let Mercedes sing, Mercedes says "Thanks Mrs. P." But as Emma is not yet married, she shouldn't have said Mrs.