Revealing mistake: Near the beginning of the film, young George already has blood coming out of his ears prior to them being slapped by Mr. Gower.
Continuity mistake: Just after George's brother comes home, George is walking across the street and a man is approaching him, but it's then a woman that passes him, not the man.
Continuity mistake: When Ernie is about to take George during his "never born" sequence, if you look at the building next to the taxi, there is the Indian Club. As Ernie is driving along around 20-30 seconds pass inside the cab. When we see outside the cab and Ernie signals to the policeman, you can see the cab has only traveled roughly 20 meters from the Indian Club.
Continuity mistake: While George is in Potter's office begging for $8,000, the snow flakes hanging around on the shoulders of George's coat should have melted away immediately after he enters Potter's office.
Continuity mistake: When George is in the Bridge Tender's office, he says he wishes he had never been born while holding a coffee cup close to his mouth, but the next quick cut of him shows his arms in his lap with no coffee cup.
Continuity mistake: Potter's rent collector is showing Potter a map of the new houses George Bailey built, but a page is folded over on the desk, covering half of an ink pen set on the desk. When the rent collector prepares to leave, that page is now a few inches to the right of the ink pen set (viewer's perspective) and hanging off Potter's desk entirely.
Factual error: When Clarence picks up his copy of "Tom Sawyer" to see if it is dry, the book is in perfect condition, despite their clothes being soaked. The pages of book that had been soaked in water would been swollen and wrinkled, and many pages would very likely have stuck together as the book dried.
Continuity mistake: Ernie hangs out of the window of his cab as Violet Bick strolls by, then ducks into his cab after the car horn blast. A quick shot later and Ernie is again hanging out of the cab's window, watching Violet walk.
Audio problem: When George is talking to Harry on the phone regarding being decorated with a medal of honor, he is at his office. The guy to the right of him starts to say "what is he saying?" but the audio cuts off just before he finishes "saying." The scene cuts to a closer shot of George talking, so why did the audio cut off?
Revealing mistake: When George comes home flustered, his kids are hanging on him. Right after Mary closes the door keeping the littlest boy out, she walks into the kitchen. Watch this scene carefully as the camera tracks backwards. A bad jump cut is obvious.
Character mistake: In every scene George is with Mary, she never once wears glasses. However, when he meets her coming from the library after he was never born, she is wearing glasses. George is shown how he has influenced all the people in Bedford Falls over his lifetime, but having Mary's eyesight degrade to the point of needing glasses isn't one of them.
Continuity mistake: In the final scene, when Mary comes into the house, she removes her headscarf, but there is still snow all over the front part of her hair that wasn't covered. When she runs up the stairs to see George, there is a quick cut to a tighter shot of them both, and all of the snow on her hair is suddenly gone.
Continuity mistake: When Violet is leaving for New York, right before she says "I'm glad I know you George", the scene cuts. This is the same camera angle as before so the scene appears to be an edit splice. The jump cut is obvious. The reason for the rough edit is that a short sequence was cut where Violet asks George to run away with her to New York. It doesn't excuse the jump, but does help explain it.
Continuity mistake: On the day before Christmas, George is showing a couple guys the newspaper about Harry being decorated by the President. The paper goes from being folded in half to open depending on the angle.
Audio problem: The cab driver says "in" twice as he's saying hop in. The scene cuts to a closer angle while he's talking. He says "hop in (cut) in."
Continuity mistake: When Mr. Martini is asked if he is renting a new house, his position in front of the chicken wire cage on his truck changes as the scene cuts.
Continuity mistake: In the scene after George and Mary's wedding, Annie suddenly appears between George's mom and Mary's mom as George and Mary make their way to Ernie's cab.
Continuity mistake: When George sits down with Mary on the Davenport in her house's parlor, her left arm is in her lap, then the scene quickly cuts, showing her left arm now resting on the arm.
Continuity mistake: George walks downtown to find Mary. He stops in the street as a man is walking the opposite direction. The man doesn't pass George at any point. The scene cuts to a closer angle and he has disappeared.
Continuity mistake: George throws a rock at the dilapidated house. He is talking about his wishes. Mary starts to turn in one angle and repeats the action in the next angle.
Answer: It was a common phrase at the time, like someone saying "oh boy!" or "hot damn."