Character mistake: When Kevin is shooting the toys off the laundry chute with Buzz's BB gun, he's looking through the scope with his closed eye.
Character mistake: When Vivian is in the tub singing "KISS" by Prince, she sings the verses in the wrong order. Not a big mistake, but a noticeable one for a Prince fan.
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: Big Ben is loaded feet first into the ambulance, he should've been loaded head first.
Character mistake: When Raphael is chasing Casey Jones after their fight he vaults over a taxi hood, never takes his eyes off Casey, but he somehow loses him.
Suggested correction: 2 things are happening; a shell backed stocky Raphael is chasing a lighter human athlete Casey. Casey has freedom of movement, whereas Raphael can't stray much from the shadows in hiding. :-) A knocked out Casey wouldn't be helping the Turtles out of the next evening with that memorable entry so I'm glad Raph lost the chase.
Character mistake: Given Billy still remembered and recognized him it seems strange Kate doesn't when she mistakes Daffy for Gizmo.
Suggested correction: Billy spent significantly more time bonding with Gizmo in the first movie - days and days. By contrast Kate had very little actual time with him.
Character mistake: When David is at Nick's house looking at his motorcycles, he says "Oh wow, a '48 FL", then asks for the key and cranks it over with an electric starter. The first Harley with a starter was a 1965 FLH.
Character mistake: When Chafuka is controlling the zombie Barry with her voodoo doll, he's supposed to be repeating her words so precisely that he even imitates her accent. But when she says "that darn fool paper, it fall on the ground," Barry repeats it as "The darn fool paper fell down to the ground."