Revealing mistake: When Henry's mom is pushed off the cliff her cable is visible.
Revealing mistake: When Henry throws the dummy off the bridge, watch the rear window of the gray car; it explodes before being hit.
Revealing mistake: In the shot immediately following the multiple car collision, there are no people either in the cars or on the road.
Revealing mistake: After Henry falls dead on the rocks, a black cable is visible that is attached to the supposed corpse of henry and to another rock ground level, possibly to stop the dummy corpse from drifting away.
Answer: Yes. She had to drop out when production was delayed, and she was no longer available.