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: 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: 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: 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: 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.
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)
Chosen answer: It was Winston. The guy John met up with in the basement of the hotel who gave John all the information he needed when he was trying to kill Iosef and Viggo.