My answer:
One surprise was when researching John Davey born in Devon around 1834.
I had travelled in England in 1990 and stayed in a bed and breakfast in Clyst Honiton near Exeter in Devon while doing that research. I began with over 50 John Davey born around 1834 in Devon. By the end of three days research, I had him down to one of seven possibilities by using 1851 and 1861 censuses using microfilms.
Little did I know then that 30 years later, I would make contact with a Davey relative through a DNA match. The match said John Dav(e)y and his parents and siblings lived in Clyst Honiton back in the 1830s.