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: 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: 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: 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)