Here is the first part of the "mistake" -
Pg. 306 George/Jake arrives in Jodie for the first time and meets Freddy Quinlan the real estate agent - "He was curious about me - I think the Maine license plate on my car struck him as exotic - but not unduly so."
And then later -
Pg. 335 Mimi casually confronts George/Jake about his identity - "you showed up in Jodie with a New England drawl in your mouth and Florida plates on your auto."
Maine plates or Florida plates? I thought about this a bit and here are the explanations I could come up with (Now I will caution those who have not read the book yet with a ***SPOILER ALERT***) -
1) Mimi was incorrect. This feels kind of unlikely as she was such an observant woman, she certainly had George/Jake pegged. I really don't see what point this would serve, other than being confusing to the reader.
2) Mimi was speaking figuratively and just meant that he had driven to Jodie from Florida. This seems like an odd way of saying what she meant, especially for a lady who was very direct with her words.
3) Reality was "shifting" due to damage caused by Al and George/Jake's time traveling. If this is the case then this is the only example of this side-effect and it was not directly addressed as such in the book. All other differences occurred during different travels while this was within the same, main, time travel episode of the book.
4) If the book is supposed to be the manuscript George/Jake wrote at the Tamarack Motor Court and buried maybe George/Jake made a mistake when he was writing. This whole setup is doubtful as the rest of the book does not really support that theory, namely it includes info about what happened to George/Jake after the manuscript was buried within the book itself. Also there is no info included about the manuscript being found, etc. In the end that is a pretty precious arrangement and kind of unlike Mr. King anyway (although quite Lovecraftian and this idea did remind me of the promotion surrounding Rose Red).