When Scrooge begins to give bags of money to Rat and Mole, there are six bags of gold in the jacket. But once he throws them all, eight bags appear. Then once Scrooge leaves, the next shot has nine bags.
The interior of the house changes, such as the location of the dining room table at the end compared to what it was like during the scene when the Ghost of Christmas Present brought Scrooge over. For example, the dining room table is near the front door at the end, but earlier it wasn't.
The front door to Bob Cratchit's house has an X on the bottom half. In the long overhead shots such as Scrooge discovering the Ghost of Christmas Present is gone and his arrival to Cratchit's house at the end, the X is not shown. Then it's back in the closer shots.
When the Mole and Rat stop by Scrooge's counting house, there's a shot of the clock where the time reads 9:00. Then when in the shot of Scrooge saying "You work all your life to get money, then people want you to give it away," the time on the clock reads between 6:55 and 7:00.
When Jiminy Cricket (The Ghost of Christmas Past) jumps onto Scrooge's bedside table, the time on the alarm clock reads 2:00. In the shot of the curtains moving when Scrooge hears the dinging, then goes back to sleep, the time reads 1:00. Then in the next shot when The Ghost of Christmas Past dings the bell on the alarm clock again, the time reads 2:00.
In the final scene at Bob Cratchit's home, the lettering on the "Home Sweet Home" sign changes between shots (particularly the word "Sweet", which alternates between uppercase and lowercase in different typefaces).
When Jacob Marley (Goofy) tells Scrooge that he will be visited by 3 spirits, he holds up 2 fingers instead of 3. This is more than likely part of Jacob Marley having the same humorous traits as Goofy.
Bob Cratchit's salary is mentioned as being two shillings and half a penny per day. In the original story, his salary was mentioned as being fifteen shillings per week. With Sundays off, that makes 2.5 shillings, or two shillings and sixpence per day. That said, "Mickey's Christmas Carol" is more an adaptation of the original story rather than a direct retelling, so there can be room for slight differences as in this case.