Revealing mistake: As Drew Barrymore is running from the killer outside he stabs her. If you look closely it is very easy to tell it is a rubber knife, because you see it bend off of her. (00:10:20)
Revealing mistake: When George Clooney is talking to Juliette Lewis towards the end of the film, it is painfully obvious that his tattoo has been smudged. (01:40:35)
Revealing mistake: After Lucy discovers Mrs. Bradley in her bed, as she fights off and runs from Patricia, it is obvious that she wears knee pads for these shots.
Revealing mistake: It's very clear they blew up a model of the buildings. The grass seen just before the explosions is way too long and is not as long when the characters are running away. Further, the truck and the barrels have disappeared at the explosion shot.
Revealing mistake: When Le Merchant is offended by his wife's comments and puts the Lament Box into the travel chest, a section of the gold veneer on it has bent upwards.
Revealing mistake: A big deal is made of the fact that the female marine is left-handed, but the shot where she throws the knife to establish this fact is flipped - the writing on the poster that she is using as a target is backwards.
Revealing mistake: When the Leprechaun gets sucked into space, he grabs onto the edges of the cargo bay's doors. You can see pieces of his hands disappear into the doors, showing that it's CG.