As a genealogist, I really detest when I find myself having to research into a name like John Smith. There are perhaps hundreds of thousands of John Smith's throughout history, which makes records hard to come by. I struggled with it once before and then several months later was going through it all again when I noticed something wrong in my ancestry database. I had duplicates and I hate duplicates. I tried deleting them only to discover that I had deleted the wrong one. I would add them in again only to find that I still had duplicates. After a couple rounds of this, I figured out the problem.
My 5th great grandfather John Smith Sr. from Virginia had five sons and three daughters. Two of his sons were named William and John Jr. William had a son named Abraham who had a son named Isaac who had a daughter Grace who was my great grandmother. I spent many a lazy summer day playing cards and dominoes with my great grandmother Grace. She could clean my pockets of nickels playing Thirty-One before I knew what hit me. She and her husband Victor were the only two of my great grandparents that I ever knew. John Sr's son John Jr. would have a daughter named Amanda who would have a daughter named Annetta who would have a daughter named Grace, my great grandmother.
So Isaac, great grandson of John Smith Sr. and Annetta great granddaughter of John Smith Sr. would marry and give birth to my great grandmother. They were 2nd cousins with their great grandparents having been dead 20 years before they were born so it is probably likely that they didn't even know they were distantly related. So I guess my great grandmother Grace was a third cousin to herself. Not everyone can say that… nor probably wants to.