Continuity mistake: When Tiger and Fievel are doing their musical piece, Tiger wipes off the mirror and they appear the same size. When he lays out in front of the mirror, singing the line "We'll be there to pick each other off the floor", he places his head next to the cage bottom, still laying on the floor between them. In the mirror, however, that cage bottom is not reflected at all.
Continuity mistake: Just before Gussie leaves Mickey O'Hare's wake, Fievel can be seen in the background behind Honest John eating a large piece of cheese from a nearby teacup. However in the next shot he's standing in a completely different location and no longer has any cheese in his hands.
Continuity mistake: After the Giant Mouse of Minsk lands in the ocean, a vacant pier can be seen from an underwater perspective. But once the angle changes seconds later, all of the mice are suddenly looking down below at the wreckage. Additionally, two ropes appear from nowhere hanging over the pier's edge.
Continuity mistake: At Mouse Garden headquarters outside of the Commission of Immigration, several vendors are trying to sell things to a crowd. Below a top hat is a sign which distinctly reads 'Mouse Garden'. But fast forward to the one offering a used ticket to Chicago right near the headquarter entrance and you'll see how it now says 'Immigration Mouse Garden' instead.
Continuity mistake: During the storm, Fievel crawls to the edge of the step he's on and watches two fish flop around below deck. Keep an eye on his blue hat. At first it's in his left hand, but between shots the hat somehow jumps below the step he's on - from which he pulls it out with his right hand and places it back on his head.