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 first meets Marcus in the cemetery, John's umbrella changes from a standard 8 rib when lowering the coffin, then a 22 rib close up, then back to 8 ribs in a far shot of the two men shaking hands. (00:04:15 - 00:05:45)
Factual error: Near the beginning when he takes his car and puppy to the gas station in New Jersey, he would not be allowed to pump his own gas.
Suggested correction: Alfie Allen also wouldn't be allowed to smoke at the gas station. Clearly the employees at this gas station are not enforcing the rules.
Suggested correction: Previously corrected. "When busy, some people are allowed to pump their own gas"
Continuity mistake: The first time John's alarm goes off at 0600, it is early moments of dawn. A little while later in the movie we see John being woken up by the puppy at 0600 and it is brighter, almost mid morning kind of light. (00:01:35 - 00:07:50)
Other mistake: When we see John's wife collapse in front of the bride, just as the camera cuts out of this little cut scene, the camera wobbles very noticeably. Almost as if someone had knocked it. (00:02:25)
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)
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)
Suggested correction: Maybe he's actually saying "Over 20 kilograms" in Russian. If you calculate the other way 60lbs is just over 27kgs.
Logically that doesn't make a lot of sense since the mistake is saying the translation says "over 60 pounds", so we have to assume by 20 he means over 27.6. Why not just just say "over 27 kg"? And you appear to just be guessing that maybe he says "over 20 kg" in Russian.
Character mistake: When he called for a dinner reservation for 12 at his home it should have been 13, as he put 13 men down. (00:32:05)
Suggested correction: Actually 14.
Actually 13. Watch and count. Many YouTube channels have covered this.
It's possible he simply miscounted in the heat of the moment. I'm not saying you're wrong was John doesn't seem like the type to make this sort of mistake but it could be let off as a character mistake rather than a movie mistake.
A character mistake is a valid movie mistake and this entry is listed as such.
Continuity mistake: Just when John Wick puts the bracelet on the night table next to the phone near the start of the film, the time changes all of a sudden from 8 to 10. (00:04:00)
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.
Continuity mistake: In the night club, when John is fighting the last guy before leaving, the guy throws John, he lands on his back, and has no gun/holster in his waist band on his back. A few seconds later a small pistol and holder appears and he takes it out and shoots at the other guy.
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: The bullet hole by Malcolm in John's pillow disappears when he rolls over the bed. (00:55:20)
Continuity mistake: As John gets up in the night near the start, the clock reads 1:41. As he is physically getting out of bed it reads 10:00. (00:12:40)
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)
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)
Continuity mistake: In the compound shootout, the one SUV gets riddled with bullets including a very distinctive double bullethole in one of the door pillars. This unique damage disappears a few shots later. (01:02:40)
Continuity mistake: The holes that Ms. Perkins puts into the glass above John's bed keep disappearing and reappearing. This is not to be confused with the extra bullet hole mistake already listed. (00:55:50)
Continuity mistake: John is thrown off the balcony in the red circle. When he lands, he doesn't have a gun in his belt at the back, but he then magically pulls one out a few seconds later. (00:50:30)
Continuity mistake: At the end, John pulls the knife out of his side and it's got blood on it. Camera shows him starting to thrust the knife into Viggo and the blood has disappeared. (01:27:40)