Revealing mistake: When Loki disguised as the delivery person grabs the baby, he's replaced by an obvious doll, with pale yellowish skin and lighter hair colour.
Revealing mistake: In the beginning of the movie, at the scene in the morgue, dead David's eyelids very slightly shake right before his casket is closed. (00:04:20)
Revealing mistake: When Walter, Danny and the astronaut are in the kitchen breaking stuff to burn, the smaller boy tries to smash an old style broom on the table. He tries three times, on his final try the shank breaks in half, revealing where it had been sawn nearly all the way through in nice clean line, with only a splintery piece showing where it really snapped.