There's Something About Mary (1998)

Revealing mistake: When Puffy the dog bites Ted and pulls him along, Ted digs his fingernails into the floor, they scrape along, making marks. If you look very closely you can see faint pre-made marks made in the floor, where he drags his nails along. (01:28:45)

Hamster

Revealing mistake: When the investigator takes Ted's head and starts slamming it on the table after Ted got finished telling them about the hitchhiker, Ted's head never hits the table but his own hand. (00:59:55)