In the first draft of a scene in Cross-Country Flying, the third-person narration changes to first person. In some cases this might cause a writer to think the whole story should be written in first person. I wrote a book once where that happened over and over again, for long passages, and I did end up changing the whole thing to first person. (The Absent Woman.)
But today, I look on the slippage as a first-draft occurrence and a good sign that I was deep into the character’s mind. When I went back to revise, I changed all the “I” references to “she,” and I’m going to leave them that way.