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)
Continuity mistake: When John's friend enters the room where he is killed he removes his sport coat. When he is tortured and sitting in a chair, his coat is back on.
Other mistake: When John is talking to the club bouncer, John says that he lost weight, to which he replies (in Russian) "Twenty kilograms." The translation says "over sixty pounds," but 20 kilograms is just over 44 pounds. (00:46:30)
Continuity mistake: After subduing one of the henchmen at his house, John sees another one come behind his staircase's side glass pane. He shoots the henchman, putting a bullet through the glass. The shot changes to him finishing the henchman, but when the shot goes back to the stairs seconds later, the bullet hole has disappeared from the glass. (00:29:55)
Continuity mistake: John stabs a guy below the chin and we see his hand get blood on it in a closeup. Next shot he pulls the knife against another guy, and his hand and shirt sleeve are perfectly clean. (00:49:30)