Revealing mistake: Right before the scene where Freddie meets with the band to try and make amends, he's arguing with that one man in the rain. After he walks away and it's raining only on him and on the pavement. If you look to the right there is no rain coming down on the grass or trees or anything else.
Revealing mistake: The paddle steamer on which Wilde crosses the Channel has her name "Waverley" visible on the bow. The Waverley, the world's last seagoing paddle steamer, was launched in 1946, 46 years after Wilde's death. Replacing the name with the name of a real paddle steamer of the era would probably have been a good idea.
Revealing mistake: When Gotti shoots the man taking a shower, he is running his fingers through his hair, but it is apparent that his hair and body are practically dry. It is also clear that the man is not actually standing under the running water, but in front of it. (00:11:00)
Revealing mistake: Callahan proudly shows to a policeman an issue of the Portland State University "Vanguard", the magazine that is publishing his comic. Looking at the prop, it is instantly suspicious because Callahan's name is all over it like he's a celebrity, and in fact, the article on it is nothing but a copy and paste of Timothy Egan's "Defiantly incorrect" article from The New Yorker celebrating John Callahan's talents in 1992. (01:12:55)
Revealing mistake: After Frank shoots Kevin and Kevin's SUV hits the curb, the background/scenery changes as well as the position of Frank's car. Frank initially pulls along the curb (behind the SUV) with his car at about a 30° angle. The next time Frank's car is shown, it is almost perpendicular to the curb and the Food Mart. When Rupple and Frank have their guns aimed, they would be aiming inside the Mart, not toward Kevin. Objects in the background also change (e.g, fire hydrant, telephone pole appear). (00:16:48)