Oz brushes his teeth almost immediately after he vomits in the hotel room. Stomach acid weakens teeth enamel, and the teeth should not be brushed for at least thirty minutes after vomiting. While this is not exactly common knowledge, a dentist would certainly be aware of the precaution.
When Oz enters the hotel room, Jimmy is sitting in a chair with his trigger finger indexed along the slide of the gun (as it should be). When the camera angle changes to behind Jimmy, his finger is instantly on the trigger (where it shouldn't be).
When Frankie Figs and Oz meet up with Jimmy in the hotel room, Frankie hugs Jimmy and he does not have his sunglasses on, the camera shows a different angle of the hug, and then when they return to his face, he does have them on again.
When Oz is trying to talk Jimmy out of killing his wife and Jill is playing with the guns on the table, the position of Jimmy's martini glass changes. From the front, it's sitting on the table, and from behind he's shown drinking from it.
When Oz and Jill are at the café, Oz asks the waitress for a burger with no mayonnaise and the waitress is seen taking Oz's menu. In the next shot, Oz re-affirms to the waitress no mayonnaise and Oz hands her the menu again.
During the sight seeing tour, Jim and Oz are on elevated view of Montreal. In the back ground is a round building, there are two men talking to Oz's right. In the close ups the two men nor the building is seen.
When Oz and Jimmy talk after first "having a drive" and Oz is sitting on a waist-high wall with a panorama of Montreal behind him, his hands switch places between shots - from down at his sides, to on his lap, and then back to his sides again.
When Jill turns on the light while Janni and his cohorts were sneaking up to the door, she had both her hands behind her back (apparently hiding her gun). In a subsequent close-up shot, she had both hands in front of her. So where was the gun then? In the next shot, she returns one hand to behind her back.
When Cynthia Tudeski visits Nicholas "Oz" Oseransky for the first time in Oz's hotel room in Chicago, she sits on the bed holding a cigarette in her left hand. When the angle changes, the cigarette switches to her right hand, then shifts back to her left hand in the next angle.
When Oz panics in his dental office and knocks over his coat rack, the coat changes positions between shots. First it is on a small hook, then it is on another hook and then it is draped over the whole coat rack.
Dental records cannot be faked. It isn't just a matter of matching up dental work on a couple of Panellipse films (as shown in the movie). There is also everyone's unique bone density/structure, unique root structure, individual tooth sizes and variations in shape to be considered. These are things that cannot be altered. Moreover, those are the things that they look at when identifying someone from dental records.
While Quebec provincial law requires the English text on any public signage to be no more than half the size of the French text, the bilingual signs with equal-size text are at the airport, which falls under federal jurisdiction.
When Oz is making the dead cop's teeth look like Jimmy's, he forgets to add or pull the upper wisdom teeth. The top x-ray shows 32 teeth, while the bottom one shows just 30 teeth. When comparing dental records this would be an instant dead give away.
When Oz is cutting the grass in the scene where Jimmy is taking Oz with him, to go on a "sight seeing", you can see that the "lever" on the top of the handles on the lawn mower is not pushed in. This has to be pushed in for the grass cutters in the lawn mower to rotate. So he is not actually cutting the grass, just pushing the lawn mower around.
The goof items below may give away important plot points.
Before Jimmy shoots Janni, another member of the Gogolak Gang breaks the 2nd and 3rd spindle up from the bottom as he goes tumbling down the stairs. But in the scene where Frankie carries is Officer Hanson and drops him on the floor, you can see in the background that the 6th and 7th spindle up from the bottom of the staircase are broken.
At the end of the movie, when the credits start to roll, Cynthia and Oz are dancing in wedding attire. When the camera pulls back to get a wider shot, it goes too far, revealing about two inches of the camera dolly on the floor. Your eyes are immediately drawn to the newlyweds, so the equipment is not obvious unless you are looking for it.