The knife Willy takes from the dish of discarded chicken bones is not the same knife that Jake was using to carve with.
When Lacey and Kevin are at the lake and Kevin says, "Mommy, mommy, I caught a fish, I caught a fish!" He is shouting, yet it is clearly Lacey doing the reeling in - you can see her shirtsleeves and it is a female adults arms and hands. Even more obviously, little Kevin is wearing a long-sleeved jacket.
Before the guy on the beach gets into his car as he searches for the breathing sound, the light on the ceiling inside the car is on, meaning the passenger side door was open for the cameraman.
Clearly fake skin when woman is stabbed with scissors in the neck.
Visible in the top right corner, as Lacey is talking to her shrink.