Stupidity: Lily dying at the end seems poetic but it would obviously cause legal trouble for Lucas and Annabel. The court/CPS would still be checking in on the girls, making sure that Lucas and Annabel were responsible guardians. There's no possible way that they could explain the disappearance of Lily. Victoria would certainly be removed from their care and there would be legal action against them, as negligent caregivers. (01:30:00 - 01:40:00)
Suggested correction: Except, of course, if Lily's body is found and her death is determined to be a tragic accident.
Her body can't be found though, because when she died, her body ceased to exist and all that was left was a butterfly.
Continuity mistake: "Mama" takes and breaks Victoria's glasses when she tells her to stop attacking Annabelle. Mama then takes the two girls, and we next see them at the cliff edge...Victoria now has her glasses back on.
Suggested correction: When Victoria is back on the cliff with "Mama" she is not wearing her glasses at all.