After Archie shoves the car jacker into the trunk, the would-be thief shoots a hole in the lid through which his blood squirts as he is made into 'fuel'. The hole, and the rag Archie uses to block it, are no longer visible when Archie throws the crippled veteran into the trunk, but reappear later in the film when he pulls out the rag so that the car will run out of 'gas'.
When Archie and Denise are in the car in the beginning of the movie, the car runs out of gas, from a few seconds earlier until that moment you can clearly see outside there is woods, but in the next shot Denise get out of the car right in front of a house.
When Archie builds the trunk machine, the blades are shown spinning clockwise. However, the serrated edges of the chopper are on the back side of the blades, making them useless.