During the assault in the library, Wick's opponent despite having the clear height advantage, only actually uses one of his arms in the latter half of the fight with it trailing behind him perfectly healthily yet completely unused, this could be down to a lack of choreography in the fight behind the scenes.
At the end of John Wick 2 when Winston gives the excomunicado order its 4 pm on his watch and he says John Wick has 1 hour.But in Part 3 he becomes excomunicado effective 6 pm which should have been 5 pm according to the ending in Part 2
The Kimber M1911 shown by the adjudicator in the continental is a full black M1911 while the one used by John Wick to kill Santino D'Antonio was a dual-tone M1911 and the one Bowery King gave to John Wick is a completely different Kimber M1911.
When John Wick meets Winston in the glass structure after being sent to kill him, several shots in which John is not in focus appear to have his ring finger in place. It's only when his hand comes in focus, do we see the ring finger has been cut.
After cutting off his finger including his wedding ring, Wick should logically have absolutely nothing left but a gap between his middle and pinky finger, however it is constantly shown that there is still a sizeable nub left over implying Wick had severed his finger at the knuckle.
At 59:13, shoot out in "Morocco Continental", Sofia is behind a column and runs out of ammo (slide is in open/empty position). When she comes out on the side, she comes out shooting with a full clip although we never see her reload.
During the library fight with Ernest, John is holding a book and using it as a weapon. When Ernest grabs John's head before shushing him, John can be seen dropping the book. In the next shot the book is back in his hand.
The Bowery King is clearly living in DUMBO. A neighborhood in Brooklyn directly across the water from Lower Manhattan. The Bowery is in Lower Manhattan and is 3rd Avenue below Houston St. When the Adjudicator tells The King to abdicate there is a clear shot of the purple F.D.R. Drive (which is located in Manhattan) behind her across the river from Brooklyn.
The scene in which John Wick fights off assassins near Chinatown in Lower Manhattan was shot entirely underneath an elevated subway line. There are no such lines in, or anywhere near, Lower Manhattan, and visible street signage indicates the scene was filmed in Bensonhurst, Brooklyn, a destination it would've taken Wick 25 minutes to reach via horseback at maximum speed instead of seconds.
Wick is shown running down Pell St. in Chinatown when visiting the doctor who sews up his stab wound. In the background of the doctor's office, numerous cars can be seen driving up and down the street in both directions. This would be impossible given that the area behind the buildings along Pell St. back up to Columbus Park, where no cars are permitted.
At Casablanca shootout, both John and Sofia fire far more shots that a pistol magazine holds and furthermore - at several instances Sofia changes the magazine and starts shooting immediately i.e. without cambering a round.
When John and his assassin are fighting underwater in the pool using automatic weapons, both are only good for 1 round. The water resistance prevents both weapons firing automatically. Instead when the triggers are pulled, only 1 round each are spent. After that the weapons would have to be cycled as semi-automatics.
The Continental Hotel in NYC is in downtown Manhattan, as established by the movie's earlier parts. However, Times Square-style lighted signage is visible during the climactic fight in the hotel's lounge. No such signage exists outside of Times Square, which would be several miles away.
At 1:53:43,when Wick pushes the katana thru Zero's chest, it never goes all the way thru since there is a hard glass floor under him. Yet, when Zero sits up, the katana has gone thru his chest and came out the back a good 5/7 inches.
The exterior stairway in the courtyard where the main fight scene takes place does not have proper steps but is simply an incline with slats where the steps should be. This makes it easier for a dog to go up and down at high speed.
When John arrives at Grand Central Station, he is met with opponents. They stab people, and walk around brandishing knives. Nobody in the general public reacts. Later, there is gun-play close to the public. Again; nobody reacts.
The goof item below may give away important plot points.
During the defense of the Continental when John Wick is using a shotgun, several times he reloads while there are still rounds in the firearm, then proceeds to work the bolt which would chamber a new round, this would eject the round currently inside the chamber.