When the police officer picks up the planted automatic weapons, he does so with his bare hands, without gloves. This would be a fatal mistake for any real police officer, as it would compromise any fingerprint evidence that could be on the weapon.
The goof item below may give away important plot points.
When Sam talks to Carla, he offers that the 4-across answer to her crossword is "BEETS". However, when Michael looks at her puzzle that she left behind, the first word is a four-letter word, so therefore the first two across clues would be 1-across and 5-across. Also, Carla told Sam that she was working on finishing the puzzle, then immediately leaves. But when Michael looked at the puzzle later, it was completely finished.