When Chris, Jamie and his Mom are watching home videos in the living room, in one of the video shots, where there is a close up on Chris' "fat face", you can see part of his fat suit peeling off his chin.
When Samantha is rehearsing her song "Forgiveness" in the jet, she asks Chris if she should use more vibrato. However, when she starts singing again, instead of using vibrato, she uses falsetto (she sings in a higher voice rather than giving the notes more vibration).
When Chris comes back to New Jersey and has lunch at the diner with Jamie, the waitress brings Jamie a grilled cheese sandwich with ranch dressing on the side. However, when Jamie trades meals with Chris and passes the plate to him, the cup that should hold the ranch dressing is empty.
When Jamie and Chris are at the diner Chris is playing with a fork in his hands. In one shot the tines of the fork are facing to the right of him and in another the tines are facing to the left of him.
When inside "the Maple", Tim tells Chris that he can have Jamie. The foam inside his beer changes from not being there in one scene and then clearly visible in the next then goes back to no foam at all.
When Chris and Jamie return from their lunch date at the diner the windows appear to be down (there is glare off the back ones, and there is no glare to indicate that the window is up in the front) when Chris returns to the car and opens the door, the window is obviously up.
At the start of the movie when Jamie and Chris are at her graduation party and they are in her room, when Jamie first jumps on the bed she is rolling around and you can see her underwear. About a minute later she is tickling Chris and leans down towards his abdomen, and her butt is in the air towards the camera, you can clearly see she has on black shorts underneath her dress. Then another minute later she leans over again and there are no shorts!
When Chris is in the dentist chair, they pop a retainer in his mouth. Retainers have to be made according to a mold of the patient's mouth, you can't just take any old retainer and expect it to fit - it won't.
When Chris is playing the game of pick-up hockey and is going for his slap shot, in all his hokey passes leading up to the slap shot, there is obviously no hockey puck anywhere near his stick. He's just faking it.
Jamie, Chris and his mother are watching old home movies after their "group date" seeing The Notebook. There's a close-up shot of "Fat" Chris saying, "I love you Jamie Palamino! You're my *best* friend". The edge of the prosthetic mask he is wearing is visible on the left side of his chin.