Factual error: When True Heart skydives down to Earth, she is wearing goggles with a wraparound strap. She takes these off and drops them in her backpack where Baby Swift Heart is hiding. When he pops up a split second later wearing the goggles, the strap is under his bonnet.
Factual error: When Fievel is being dressed by the pigeon there are two American flags in the background. The flags have too many stars for the mid 1880s, when the movie is set.
Factual error: Apparently, Ratigan had to cross the River Thames to reach Big Ben from Buckingham Palace as evidenced by him tossing Fidget into the river before taking reign of the blimp. However, the two landmarks are on the same side of the river.
Suggested correction: Ratigan changed direction multiple times during the chase because he was trying to get away from Basil. He may have flown away from that side of the river, but then after changing direction he flew across to the side he came from again.