Continuity mistake: In "Wheels," when Artie is singing 'Dancing By Myself' and doing tricks in his wheelchair, the wheels on the back of the chair that prevent him from falling backwards disappear and reappear, depending on whether or not he is performing a trick.
Continuity mistake: During "Theatricality", when the boys are singing "Beth", just as Puck sings "Me and the boys are Playin'" for the first time, Finn is visible sitting behind Quinn. Yet Finn went to sit down and sing with Puck and is obviously sitting with the boys in the next wide shot.
Continuity mistake: In "Hell-O", as Santana and Brittney are talking about the "hottest guys in the school", Santana's glass is almost empty. Yet two shots later, when she takes a drink, her glass is full. Also, when she says "Would you mind waiting in the car?" her glass is full and you can obviously see a slice of lemon in it. Yet in the next shot, when she says "And leave your credit card", her glass is only about 1/3 full and the lemon slice is gone.
Other mistake: When Finn and Rachel sing Smile in the Choir Room in Mattress (Season 1, Episode 12), Finn doesn't appear to look at any other page but the first one in the music given to him by Rachel. It is unlikely that the entire score for the song was on the first page.
Continuity mistake: During "Don't Stop Believing" at the line "He took the midnight train going anywhere," Rachel extends her hand with her palm facing the floor. In the next shot, it instantly changes to her palm facing the seats of the auditorium. (Timecode listed for the director's cut of the episode.)
Continuity mistake: When Mercedes is singing "Bust Your Windows", the blue rags on Kurt's car behind her change position several times. Also, Kurt is at first looking up at the hole in his windshield, then has his head down on the hood, and then is looking up at the hole again, over a series of 3 or 4 shots.
Continuity mistake: In "The Power of Madonna", during the Cheerios stilts routine, as the people on stilts lift one leg one after the other, they are in a V shape, and there are both girls and boys on stilts. Yet in the very next shot, as the people on stilts swing the other Cheerios around, there are only 3 Cheerios, all guys, on stilts, and they are in a straight line.
Continuity mistake: When Artie tries to walk with crutches after the "step dance" practice with Tina, he falls down violently. Immediately after the fall, the crutch is still attached to his left arm, which is angled away from his body. In the very next shot, the crutch is almost a foot away from his arm, which lies now at a different angle.
Continuity mistake: During "Home", when Mercedes is singing "Beautiful", Quinn stands up to join her. When she passes behind Mercedes, she stops and puts her hands on her stomach. Yet in the next shot, she passes Mercedes a second time and once again stops with her hands on her stomach.
Continuity mistake: During "Bad Reputation", as the kids are singing "Ice Ice Baby", their sheets of music continually change position, especially with the people sitting down. For example, in the first shot Mercedes is holding her music in her right hand. In the next shot it's down on her lap. In the third shot she's holding it with both hands, and in the last shot it's in her right hand again.
Other mistake: In the scene at the lockers with Brittany, Santana mentions having trouble finding a younger man per Sue's orders. However, Santana is a senior in season 3, which would make her a sophomore in season 1. Therefore, she could easily just date a freshman instead of needing Finn.