In the scene where David is going to confront the villagers about the cat, he lets in 4 men, but when they gather in the living room, there are only three villagers.
The men open the mantrap for display then ask where to put it. They are asked to hang it above the fireplace. The guns which were hanging there whilst they opened the trap have, in the meantime, disappeared.
After Amy tells David to get some lettuce, David notices the addition symbol on his chalk board changed to subtract by Amy earlier on. When he draws it back on, the symbol before it jumps from being an addition to a subtraction symbol.
When Amy fires the shotgun at the last attacker both the hammers are in the 'uncocked' position. She would need to pull the hammer of the relevant barrel backwards to cock the gun.
At the start of the film Dustin Hoffman goes into a village pub and asks for American cigarettes and he's give Rothmans, which are British.It would be highly unlikely for a village pub to sell American cigarettes.
In the scene where David is taken duck shooting, he fires his gun into the air at ducks flying overhead. We see ducks flying to the right and straightaway to the left. It is the same film reversed.
As David and Amy Sumner drive up to their house for the first time, the camera crew is reflected in the car's side window.