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)
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: 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 from Marcus' shot at John's pillow disappears when he rolls over the bed. (00:55:20)
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)
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)
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.
Continuity mistake: Once John has been knocked to the floor in the compound scene, he has his hand next to his face. Camera changes and it's further away. (01:03:50)
Continuity mistake: In the red circle, John stabs a guy in the shoulder before shooting him. When he shoots him however the knife has disappeared. (00:48:40)
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)
Continuity mistake: When John is going to bed with Daisy for the first time, the clock says 10:50. John puts a pillow on the floor and now the clock's showing 10:54. (00:08:30)
Continuity mistake: At the end, John pulls out alongside the two trucks. Camera changes and now he's back in a single line behind the trucks. (01:22:34)
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.