Deliberate mistake: When the kids are about to jump start the trap Phoebe hides behind the school bus door. Half a second later, when the trap opens, she's nowhere to be seen, not even jumping or stepping inside the bus. Then half a second later she has managed to get inside, turn around and protect her head while the bus windows burst. It all happens in real time for their hairs are still fluttering. It sure paces up the scene, but it's really awkward.
Deliberate mistake: A shot of the pile of ashes from the fire wherein Cassie's body was essentially cremated showed Nina's half of the broken heart necklace sticking out ABOVE the ashes, which would be impossible. Also, a metal charm on a chain that was around Cassie's neck would not survive the huge fire or be totally clean and unscathed.