Deliberate mistake: When the vortex is summoned to get rid of the monsters, other things get sucked into it. Cars, trees, light poles, other police officers. However other things or people who are more accessible do not get sucked in. For example, Phoebe hiding behind a bench, the bench itself, Phoebe's mother and a few cops hiding behind a cop car, etc.
Deliberate mistake: While the rock n' roll musical is ending, it shows the hot air balloons in the background and it shows Dave up close. He talks to himself confused, trying to figure out if he really saw his boys there or not. But if not being able to notice, wouldn't he at least hear them singing in the background as well as we can? Also, before, while Alvin and Britany were dancing together, he was trying to get to them before the two bad guys could, as if he knew for sure they were his boys and that those guys were out to get them.
Deliberate mistake: When the hillbilly is dragged into the cellar, the resulting explosion of blood has a much greater volume than the actual amount of blood in a human body. This is done to support the campy gore motif of the film. (01:03:20)
Deliberate mistake: When Jonathan and Emma are beginning to know each other, they make their way to the wood shop, where Emma picks up an "air powered tool". The tool is a staple gun, but shoots two 3" nails close to Jonathan.
Deliberate mistake: When Larry comes inside the house (right after Kirsty encounters the cenobites and Frank and Julia discuss getting him a new skin), he is absolutely bone dry... but we can hear a steady, heavy downpour outside when he opens the door. Seems like they added a rain sound effect in post for a creepy ambiance even though it wasn't shot with that in mind. (01:09:35)