Continuity mistake: When Alex walks forward in the cafeteria with his notepad, he stops by the cafeteria tills. In one shot, you see a student going up to pay for their lunch. But it then cuts, and the student has vanished.
Continuity mistake: Charlie's tie at the start of his first day as teacher is green, beige and white, but at noon when the Sheriff comes in to check on him, his tie is now white, brown and yellow in an entirely different striped pattern. After school the original tie is back on. (00:31:00 - 00:37:00)
Continuity mistake: When Marie is in the back of the van taking the chains of Alex, Alex picks up the knife and it is obviously bent. Once the chains are off and Alex puts the knife up to Marie to defend herself the blade is straight, and stays that way even when "the killer" takes it out of his chest.
Continuity mistake: Near the end of the film when Ray and Hilda are having a conversation, Ray's white undershirt alternates between being seen and being hidden under his shirt (on his shoulder).
Continuity mistake: When John Gacy hands a joint to the young guy he's picked up on the corner, the joint is nearly finished. When the young guy hands it back, it's almost a full joint.
Continuity mistake: When Henry is down after being shot, the blood pools by him are constantly changing size. (00:46:50)