When Molly goes through airport security, she is asked to remove her belt before going through the metal detector which she does. In the next shot (going through the metal detector) she is wearing it and the next shot, she it putting it back on.
When Kirk & Molly have their fight after Katie's party, Molly's robe is tied differently in the bedroom than it is when she stands at the door Kirk just left through. She was not shown retying the robe during the scene.
At Katie's 21st birthday party, Stainer removes his jacket after finishing "Kiss on My List". His shirt is clearly not tucked in. Moments later, while singing "Everytime You Go Away" his shirt is tucked in. Seconds later, he can be seen behind Molly and Kirk and his shirt is again not tucked in.
When Molly and Kirk are at his parents for dinner, she assures him that she's not going anywhere and grabs his leg under the table, but as the camera angle changes, her hand is placed firmly on the table.
Just before the altercation in the bowling alley, Stainer puts his beer on the scoring table. After things cool down, he sits down without picking up his beer. While sitting and in a continuous conversation that does not show him getting his beer or someone handing it to him, the beer is seen on the scoring table in one shot and then suddenly is in his hands two shots later. In subsequent shots while they're debating Kirk's hotness number, Stainer's beer bottle is alternately seen both in his hands and on the scoring table.
When Dylan is at the edge of the pool, there are two beer cans on either side of him. In the front angle, the can on the left is sideways and you can see the oval logo on both sides of the can. However, in the back view in the next shot, the can has suddenly rotated 90 degrees with nobody touching it: one of the oval logos is now facing the camera. The logos should have been on the sides whether viewed from front or back.
When Cam comes up to Kirk at the air show, during a continuous conversation, one shot shows Cam's hand completely on Kirk's right shoulder but the next shot shows Cam still reaching across Cam's back to grab his shoulder.
In Katie's party at the Brillobox, Kirk is wearing a dark undershirt under his long-sleeved dark shirt. But in the room scene right after the party, when Kirk takes off his long-sleeved shirt, he is no longer wearing the dark undershirt.
When Marnie gets off of the plane and is chasing Kirk, she takes off a pair of red wedges so that she can run faster. After their ride on the cart where he pushes her off into a small crowd of people, her feet can be seen sticking up from the mass on the floor and she is clearly wearing black character shoes.
In the end scene, the small airplane is cleared for take-off from runway two-niner. This runway does not exist in Pittsburgh. Pittsburgh has runways 10R/28L, 10L/28R, 10C/28C and 14/32. Also the L (=left), R (=right) or C (=center) should have been added during the transmissions.
In the scene where Kirk is asked by Cam to put in a "good word" for him, the patch on Cam's flight suit is actually that of the Canadian Air Force 431 Air Demonstration Squadron - aka "The Snowbirds", not an American unit.
At the hockey game, Molly points out that the forward is playing back because the other team is on a power play. Kirk then looks up at the scoreboard and it shows that the power play has just started (since there are 2 minutes left) which means that during the previous play, Molly was wrong in saying they are killing a penalty.
At the very end of the film when Kirk takes Molly on the small plane, he radios the tower asking for takeoff clearance, the tower responds with the callsign and "cleared for takeoff" without including the wind direction and speed. Although commonly given, this is not required of the tower controller as this information is available on a separate weather radio frequency which pilots are required to listen to before contacting air traffic control.
After Kirk makes his speech on the plane and the flight attendant won't let him off, the flight attendant tells him the flight is full and he needs to get back to his seat. Shortly after Molly arrives, a ticket is issued to her for that flight by his friend Devon. Logically this is not possible, but as anyone who has traveled by plane knows, airlines regularly overbook flights. It is likely Devon used this loophole to issue a ticket to Molly.
Towards the end of the film when Stainer is trying to get Molly onto the departing plane to meet Kirk, he is pushing through some departure lines when he suddenly makes a roar of triumph or despair with his voice. A blond woman in the line clearly is seen really trying to stifle a laugh though she wasn't being very successful at it. As an extra in the film she probably knew the shout was coming but couldn't stop from laughing.
In the "sex" scene in Molly's bedroom during which Jay Baruchel has his shirt off, you can clearly see cover-up makeup to hide his maple leaf tattoo over his heart, because the shade of makeup does not match his skin tone.
When Kirk and Molly walk in the house, Mr. K is watching TV. He and Mrs. K get up to greet the visitors and the TV volume goes down even though nobody touched the TV or a remote. And it's not just distance from the TV because when the volume goes down, you can hear the grandfather clock ticking and that's farther away from the action than the television.
At the end of the film when Kirk is on the plane to Branson with his family, the external shots of the aircraft confirm it is an Embraer 190, however, the shots taken inside the aircraft show the plane having 3 seats on each side of the center isle, this is incorrect as an E-190 has only two seats on each side, proving a different plane was used for the external and internal shots.