Sometimes a clue is not a clue the first time you see it.
I had used a deed as a sample in my early years of teaching genealogy classes. After a few years, I switched it out in place of a different example. Several years later, I switched back to the earlier example, not really reading it but just putting it in.
I read again as I lectured about it and then I stopped. The purchaser of the land in question was an ancestor--the reason I had copied it. Now years later, I stopped and looked at the name of the seller. It was my ancestor's first cousin who had "evaporated" in Ohio. Here he was in Illinois selling land to my ancestor.
Now I know to look closely at all the names on any deed where an ancestor buys or sells property. I didn't know that when I was starting my research.
How many things did you find early in your research that have not been re-analyzed?