Continuity mistake: When Dustin Hoffman drives into town at great speed, he swerves around a tractor going in the opposite direction. He parks the car and exactly one minute later the same tractor drives towards him from the middle of town and heads off up the same road it had already travelled. To accomplish this, it must have turned around and driven back to town at about 90 mph - about 5 times its normal top speed.
Continuity mistake: When David enters the living room to see how the men are getting on with the man trap, you can see there are two rifles hanging above the fireplace, but when David tells the men to hang the trap over the fireplace, the rifles have disappeared.
Revealing mistake: When David is re-loading his gun whilst out duck shooting, we see three ducks flying to the right and then immediately to the left, revealing that it is the same film in reverse.
Continuity mistake: When Amy tells David to go out and get some lettuce, David notices the addition symbol on the blackboard was changed to subtract by Amy earlier on. At this point, there are two subtraction symbols between the equation, but in the next shot the left subtraction symbol has changed to an addition, and in the shot after that, it becomes a subtraction again.