My kids, when learning to talk, have all made pretty much the same grammar mistakes. Drived, sleeped, putted. Using object pronouns for subject pronouns. Not using the adverbial -ly properly. Etc. But Violet has been regularly using one that I haven't heard yet. She's taken the words am and not and created the contraction amn't.
Example:
Violet, are you being naughty?
No! I amn't!
I find a lot of these grammar accidents fascinating because they are usually arrived at logically and you can see how the child has internalized so many of the rules just not all of the exceptions. But other times words and sentences come completely from left field and that's even better!
What are some good ones you've heard?
