When Rachel, Kurt, Mercedes, and Blaine are at the Lima Bean, you can first see that Blaine took the top off his coffee cup and is dipped his pastry in it and at it. However, in the next scene you see the pastry uneaten and where it was before and the coffee cup unopened as he drinks from it.
When the football game is starting, the scoreboard says that the visiting team has 17. Since the game is just starting, this would be impossible. However, we see later on that they do in fact have 17 points.
In the episode Mash-Up (2009) Dave Karofsky says that he is part of the Hockey team. However, in this episode you see him getting slushied by the Hockey team.
Before Sue catches them in the bathroom, Quinn mentions that failing to show up for the halftime show will mean they'll be kicked out of Glee. However, Mr. Shue never says this is the case, just that they'll be missing out on the iconic song choices. Before Sue forces them to quit, there's no reason Quinn, Santana and Brittany couldn't have just missed the one performance but stayed in Glee.
During the football game, when Tina gets tackled, the athletic trainer would never roll someone over without confirming she was unconscious and properly bracing their neck. And since Tina hadn't said anything at that point, they didn't know if she was capable of moving.
When Brittany is signing the consent form so she can be fired from the cannon, the form clearly states that a parent or guardian must sign for a person under the age of 18. Since Brittany is only a Junior, she's under 18 and thereby cannot sign the form herself even though she does.