One of mine came when I had already discovered that one of my third great grandfathers, August Bauer, had owned more than one property that he rented out and I was looking for the names of his tenants. After all, these people had been a part of his life and therefore I felt they were worth looking up. I wasn't exactly going to research their life stories but some names and perhaps some other basic details would suffice.
I accomplished this by using Ancestry.com's City Directories (Beta). The "beta" means that it was indexed by OCR (optical character recognition) software. Basically, a piece of software is programmed to recognize the shapes of all typed characters in a scanned image and converts them to real text that can be searched. The advantage is that it can process thousands of names quickly and easily. The disadvantage is that it's not always as accurate as a human being and is heavily dependent on the quality of the scan.
So I was aware that due to OCR errors, I probably wouldn't find all the tenants who lived in my ancestor's houses but I wanted to find what I could. Using the directory search, I left the name fields blank, I put "Pittsburgh, Pennsylvania" into the "Lived In" field, entered a mid-range year for when August Bauer owned his properties and then added "+/-10" years under "Any Event". Lastly, and most importantly, I put one of the addresses of August's properties into the "Keyword" field. With the results, I checked each one to make sure they were the correct address and time period and then I wrote down their names and occupations. I repeated this for each address.
In the midst of this, I spotted a name that immediately made my heart jump. I rushed to refer back to my family tree (and by that, I mean I moved my mouse and clicked as quickly as I could) and sure enough, there he was: Robert Russell. I realize that name means nothing to you. Let me explain. Robert Russell was the father of Anna Jane Russell who coincidentally married the son of August Bauer!
How could I have missed this?! The directory record was from 1899 so surely, I would have spotted Robert living right next to August in the 1900 censuses? I previously couldn't find a census record for Robert, for reasons I would soon discover. I hurriedly opened August's 1900 census record and sure enough, there is Robert listed right below the Bauer family. How could I have missed this?!
I had overlooked Robert because his daughter was already married and living with her husband by this point and the woman I had for her mother was named Catherine, while this Robert was living with his wife Annie (as it turns out, Catherine died and Robert remarried but I didn't know that at the time). So in my searches, I had no connection between this Robert and my Robert apart from the name, which isn't enough to go on. Once I realized he was renting a house from my other ancestor, everything started to fall into place.
Moral of the story? Look at your ancestor's neighbors!
What "How could I have missed this?!" moments have you had?