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.