Revealing mistake: Just as John grabs the dog after being attacked, the dog is added post production. This is identifiable as his little finger goes through the dog. (00:14:00)
Revealing mistake: In the compound shootout there is one shot of John hiding behind a SUV, with bullets hitting all up the side of the truck. Look at the glass to see 2 explosive patches that turn into bullet holes. (01:02:35)
Revealing mistake: When John is sent flying in the compound it's clear to see that John is replaced by a stuntman. (01:04:00)
Revealing mistake: When John finds Marcus dead on the floor, you can see Marcus is still breathing. (01:20:50)
Revealing mistake: When Viggo pushes the car off the edge and it hits the ground, as it rolls you can see that the engine/transmission, drive shift and tank have been removed. (01:26:00)
Revealing mistake: When John enters the airport to practice his driving skills, and is driving down the parking lot next to the taxiway - right as he makes his very first 180° turn to skid his car around, looking closely you can see the front tires never turn. It is impossible to intentionally skid a car to do a 180 without turning the front tires. Yet he does exactly that.
Revealing mistake: During the home invasion scene, the final guy is stabbed by John. On the 2nd and 3rd stabs the knife can be seen to flex slightly as John hits it.
