Continuity mistake: When Pnub's head is picked off the kitchen table at the end of the hand-cutting scene, the ring of blood that formed around Pnub's neck stump is joined with the larger pool of blood that it came from. When Pnub and Mick come back to the kitchen later on, the ring is no longer attached to the larger pool. Blood does not evaporate like that when it dries.
Visible crew/equipment: When the mom goes downstairs to see what the sound is, she slips on blood and falls on the floor. When she starts to stand back up, you can see her knee pads.
Continuity mistake: The cheesy puffs that are thrown to the decapitated head in the chair change from scene to scene.
Factual error: When the bloke's head gets cut off in the beginning by the circular saw blade being thrown, surely there would be more blood, like when his mom got killed under the bed?
Continuity mistake: When Anton is going to the dance and is backing out of the driveway in the pickup truck, you can see both of his hands on the wheel. This is after he has cut his hand off.
Continuity mistake: At the very beginning of the film, Anton's mom and dad are lying in bed together talking. Right after Anton's dad explains to his mom how "Anton never even helped me with the Halloween decorations", he turns the light out and lies there with both of his hands behind his head. A split second later, Anton's mom looks up at the ceiling and sees the words "I'm underneath the bed" glowing on the ceiling and begins screaming. The camera pans back to Anton's dad, and both of his arms are at his side.
Audio problem: During the scene right after Anton runs into the scarecrows in his house only to find out they are his deceased mom and dad, you see him run out to his porch and begin throwing up into his bushes. You can hear the sounds of liquid hitting the bushes, but you see nothing coming out of his mouth (there is enough light in the film to determine this).
Revealing mistake: During the scenes at the Halloween dance when The Offspring are playing their two songs (I Wanna Be Sedated, and Beheaded), while some of the vocals and drums may be live, it is easy to tell (especially for a musician) that many of the guitar strokes are being 'mimed', and would not match the actual chords to the song(s) parts.