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: 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.
Continuity mistake: At the beginning in the soda shop, the first little girl (Mary) is facing toward the camera, away from the other girl that comes in. When they start talking however in the next shot, both are facing outward.
Answer: It was a common phrase at the time, like someone saying "oh boy!" or "hot damn."