Continuity mistake: When Piper reveals herself as the killer in the dock, you can see her dropping the mask and shortly after, removing her poncho and drop it on the floor as well. In the next shot from the side of the dock where you can see Piper and Emma face to face, the mask and the poncho are no longer on the floor.
Continuity mistake: When Eli tries to cheer up Emma, the house they arrive at has a sign outside that says "Wren Lake Estates Sales Office." A few minutes later, there's an exterior shot of the house and the sign suddenly says "Wren Lake Estates Model Home." Once the house catches fire, the sign says "Sales Office" again.
Other mistake: In the third scene, where Emma is texting Kieran, the time stamp on the text reads 3:32 PM. She then goes downstairs where her dad is waiting and says she's "late for school." If that timestamp was accurate she'd have missed almost all of school.