Character mistake: Tom states that the Republic of China released fifteen American POWs in 1955. The People's Republic of China (Communist China) might have done so, but the Republic of China (Taiwan) has always been an American ally and would not have been holding any Americans prisoner to begin with.
Character mistake: Big Ben is loaded feet first into the ambulance, he should've been loaded head first.
Character mistake: Ashley should be smart enough to know not to wear high heels while buffing the bowling alley lane. They might break or scuff up the floor, she should've worn sneakers or sandals.
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.