Character mistake: During the movie the cat Winky is referred to as male, but much later in the movie is referred to as female.
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.
Character mistake: When we see what is happening inside the saloon, there is a sign which reads "Noisy - Is'nt It?" It should read "Isn't."
Character mistake: When Jack offers to sell his cow to the butcher, the butcher says "There isn't much meat on him." Jack points out that the cow is a "her", but after the sale, he says to the cow, "Be a good boy."
Character mistake: At the ravine, upon learning that the bridge doesn't exist yet, Marty states to Doc "We can't wait a year and a half for this to be built". If you carefully look at the sign, it says the bridge is opening Summer 1886. That's under a year away, not a year and a half from Sept. 1885.
Character mistake: When Dracula puts Johnny on a time-out, it is shown that Dracula has the ability of psychokinesis. Yet during the scene when Mavis visits the human village, when one of the dressed up zombies loses its head, Dracula jumps into the scene to physically put the head back to its body, further risking being spotted by Mavis, which he wanted to avoid. (00:16:15)
Character mistake: When Jernigan chases the car, he tells the team, "It's heading south on Adams." The toy car then turns left, which means that the car should be heading east. However, Jernigan tells the team that the car is headed west.
Character mistake: On the sign outside Dr I.Q. Hi's headquarters, the word "Stratosphere" is spelled "Stratopere."
Character mistake: When we see the sign in the window of the music shop, it reads "This Week Dance Record" which doesn't make much sense. Surely it was meant to read "This Week's Dance Record."
Character mistake: After the dodgeball game ends Jerry asks the referee if skyzone had a bar, but in the background he's already walking away.