Deliberate mistake: The entire plot wouldn't exist without this one, but just before the engineer stumbles out the door and falls off, he pulls the big red lever on the left side of the cab. In the real world this puts the air brake system into its emergency mode. Among other things, this mode trips a solenoid which causes the engine to return to idle, and the electrical system to not put out power. Neither the Diesel engine nor the electrical system will respond to control inputs until/unless the air system is reset.
Deliberate mistake: The mirror that made Rocky's face look normal should've made his mom's face look different, but her face stayed the same.
Deliberate mistake: David spots the boat with the old folk on and runs around the harbor, down the jetty and jumps towards the dive boat as it leaves the jetty. He doesn't jump very high and you can see he would miss the rail on the boat. In the next shot, a (larger) stunt double is shown leaping high in the air onto the boat and, of course, catches the rail.
Deliberate mistake: During the finale of "One," near the end of the number when the entire cast and all of the extras, dressed in gold lame tuxedos, are doing the last kick line before the credits, one of the dancers (second row, third from the right) falls down and gets right back up again. While this mistake wasn't "deliberate" as such, it was probably simply prohibitive to film the entire kick line over again for the sake of the one fall that is barely noticeable, so was deliberately left in.
Deliberate mistake: To keep things simple, the movie relies on the Lancaster County Sheriff as a county police force. In reality, the county sheriff serves the county court (transporting prisoners, for example) but does not serve as a county wide police force. In PA, each city, borough, and township has its own police force. Note the varied colors of police cars at the farm in one of the final scenes. These were borrowed from the various local police forces (Lancaster City, for example, is robin egg blue with a white top).