Other mistake: When Phoebe arrives back from her vision quest to find Piper has been kidnapped she tries to sense from the frozen students which one created the headless horseman. As Gideon, Paige, Chris and Phoebe walk into the conservatory you see Zachery blink rather obviously.
Other mistake: In previous episodes it is stated that good witches don't freeze. But after Piper has been decapitated and her head is placed in the manor, one of the students freezes all the kids by tapping into Piper's freezing power. The kids are all good witches so they should not have frozen.
Continuity mistake: When Phoebe comes out of her vision quest the third time, the shaman holds out the potion cup by the handle, but in the next shot, she's holding it by the bowl. The cup also appears to jump from her right hand to her left.