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)
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.
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.