When Mary Kate interrupts the father fly fishing after storming off, the father says that he has been fishing for a particular wily salmon for ten years. Salmon almost always expire after returning to fresh water to spawn (at an age of four years or so) and a keen fisherman would know this.
Michaeleen introduces Sean to Mary Kate as from Pittsburgh, Massachusetts, USA. However, "Steel", "pig-iron furnaces", and "slag heaps" are more generally associated with Pittsburgh, Pennsylvania, USA. (Note: this is an obvious comedic device, showing that Michaeleen is not familiar with US geography.)
When Thornton and Danaher are at the Widow Tillane's bidding for the cottage, Danaher's highest bid is 710. But in the bar, the accordion player says he heard that Danaher bid up to 810 and nobody corrected him. It is possible that the bidding was longer when originally shot, and edited down, but they forgot to change the scene in the bar.
When Mary Kate and Sean drive to town in the buggy, the horn is on the left hand side of the bulkhead. When Mary Kate interrupts the father fly fishing after storming off, it has moved to the right side.
Towards the end of the movie when all the town folk are along the road waving at the car passing by, the car twice passes by the "IRA" member. He can be seen standing on the side of the road in his bright blue shirt.
When Sean steps off the train in Castletown, he lays his sleeping bag down first and a suitcase down to the left of it. When Michaleen walks up to retrieve them, the sleeping bag should be on his left and the suitcase on his right; instead, they are reversed when he picks them up.
When Sean visits Rev. Playfair, the vicar opens his scrapbook to the page which shows the newspaper article declaring "TROOPER THORN QUITS RING. Heavyweight Challenger Hangs Up Gloves After Fatal Knockout. Vows he will never fight again." The article goes on to say: "Trooper Thorn declares he has quit the ring for good and has hung up his gloves for the last time. Thorn's last fight was a knockout over Tony Gardello and so decisive was Thorn's victory that it will be some time before Tony can resume the fight game..."
Before Sean enters Mary Kate's home to ask her brother's permission to court her, the flowers he's carrying are very sad looking. After he enters the house, they change into a nice, full, colorful bouquet.
During the wedding reception when Will Danaher makes a toast to himself, the Widow Tillane can be seen in the background during the wide shot with her hands together holding a glass. During her close-up, her hands are apart, resting on the chair's arms.
When John Wayne and Maureen O'Hara are first driven along the edge of the water, by Michaleen Oge Flynn, (Barry Fitzgerald), John Wayne offers to give her a lift off the trap, but Maureen O'Hara decides to jump down unaided. As this is going on, one can see the backdrop of Lough Corrib and its many and distant islands, beyond. John Wayne and Maureen O'Hara then start to walk in front of the trap and the camera changes to a different view, which shows them walking by the river in Cong, barely twenty feet wide.
Some of the pipe tunes which are played are Scottish rather than Irish. Whilst it is not uncommon for Scottish and Irish musicians to play each other's tunes, the Quiet Man is clearly about Irishness, and so this is no doubt an oversight.
In the race meeting scene, two pipers are shown playing. The pipes they are playing are Scottish pipes, characterized by having one bass and two tenor drones in addition to the chanter; traditional Irish pipes only have one bass and one tenor drone, plus the chanter.
After Sean and Red's first confrontation in the pub, the publican is pulling a round of drinks for everyone. It can be seen that he is taking full pints from a line and pretending to fill them. In Ireland, it is customary to have a row of drinks, such as Guinness, ready to be topped up as it is best served once it has settled.
When Sean finds Mary Kate cleaning his cottage, you can see in the background that the bedroom door has been splintered with a large hole where the bolt would be. The door isn't splintered until later in the story, though, after Sean and Kate are married and he kicks the intact bedroom door down.
Whilst on the formal courtship along with Michaeleen, they were doing a walking stretch. Sean asks Mary Kate if she can ride a bicycle, as Mrs. Playfair's tandem is seen sitting idle in town. She answers yes and the two take off on their own, but Sean should have already known, as the two were shown earlier accidentally meeting on the road; he on his horse and she riding a bicycle.