Continuity mistake: Outside of Ma Bailey's Boarding House, Clarence is leaning on a mail box, holding his copy of Tom Sawyer. The position of the book changes in Clarence's hands as the shots quick cut from him to George Bailey and back.
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.
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.
Continuity mistake: In the scene where Violet and George talk about going up to Mount Bedford, they start their walk down the promenade, close to the intersection where Gowers Drugs and The Bijou theater are visible in the background, along with a tree with no visible branches and a Caucasian couple seated on a park bench facing screen right. At the cut to the closeup, they have moved about 50 to 75 feet further down the promenade, the drug store and theater are no longer visible, the tree behind them now has a branch, the couple is now African American and the park bench is facing screen left. (00:41:30)
Continuity mistake: After George retraces Billy's steps before he went to the bank, George yells at Billy that he will not be the one to go to jail, and when Billy starts sobbing his right arm is over his left in the wide shot, but in the semi-closeup it's the opposite way, with Billy's left arm over his right. (01:24:20)
Continuity mistake: After George shows the bank examiner the newspaper and tells him about Harry's honor, George leads Mr. Carter to his office while holding the pipe in his left hand, but it cuts to the semi-closeup and the pipe is suddenly in George's right hand. (01:18:45)
Continuity mistake: While Clarence and George are at the Pottersville cemetery George asks about Mary, and in the shot facing Clarence he's holding his book upside-down with its spine facing him, but it cuts to the closeup and now the book is right-side up, with its spine facing the opposite way. (01:59:20)
Continuity mistake: After George rails at Uncle Billy for losing the bank deposit, when George gets home he removes his scarf and Mary tosses it onto the table beside the telephone. The scarf is still there when George and Mary head to the kitchen, but then it's suddenly gone and it's not on the floor either. Also, Pete jumps off the chair and is behind his parents, then two shots later jumps down again from that chair. (01:25:10)
Continuity mistake: While George is sprinkling coconut on Mary's ice cream he notices the telegram, and when he picks it up we see the lines of the message as well as the folds in the paper. In the next closeup the lines of the message and the paper's folds are entirely different. (00:07:15)
Continuity mistake: When Billy is at the bank to make the deposit he writes the date on the slip, and as he starts to take the cash out of the envelope he's still holding the nib pen between his fingers (behind the inkwell), but in the closeup the pen is gone, then in the next shot it's back between his fingers. (01:19:00)
Continuity mistake: After Clarence saves George from suicide, while they're both inside the bridge tollhouse, a photo suddenly appears tucked behind the thermometer between the two windows which was not in the first interior shot, and then it keeps disappearing and reappearing during this scene. Other changes occur within these shots, such as the window shade to the left of the door and Clarence's book position on the line. (01:39:50)
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."