Revealing mistake: Mama's eyes twitch as Arnie is bouncing up and down on her bed, even though she's supposed to be dead. Later, when the kids are around her in a vigil, she is very obviously breathing.
Revealing mistake: When Gilbert finds Arnie in the bath and takes him out, you can see a towel around Arnie's waist.
Revealing mistake: During the rainstorm, after Gilbert delivers Becky and her groceries back to the camper, they are embracing in the rain and hoping that Nana won't get the camper started. You can see sunlight reflecting off the truck, their hair and their shoulders, even though they are standing in a supposed downpour with a completely cloudy sky.
Answer: They shot it last and they didn't wear wigs.