When Artie is dancing in Safety Dance, there is a J. Dew store behind him. In between shots the store goes from being opened to closed. This happens various times between shots.
When Will Schuester and Bryan Ryan are singing "Piano Man" in the bar, Bryan's bottle of beer disappears and reappears between shots.
When Artie falls in the choir room, his glasses fly off and land under his right arm, but the next shot, from a different angle, has him with glasses on.
Rachel Berry says she was born on December 18, 1994. She would technically be a freshman in high school at the time.
When Will and Bryan Ryan are singing Dream On, camera men are visible on both sides of the stages above them.
When Artie is dancing in Safety Dance, a camera man is visible on the second floor of the building behind the crowd.
When Artie is dancing in Safety Dance, there is a J. Dew store behind him. There are no J. Dew stores in Ohio, only in California where filming takes place.