When I was growing up, we were cheek-kissers in our family, but my first cousins were lip-kissers. My grandparents (cheek-kissers by nature) had adapted to kiss them the way their mom did.
One day when I was about middle-school age, as I was leaving, my grandpa got mixed up and kissed me on the mouth. It was just all kinds of wierd for both of us. I felt so bad for him, he was so embarrassed.
The funny thing is, I kiss my kids on the mouth now. They always saw me and DH kissing like that, and started coming at me lips-first as toddlers. I just didn't have the heart to turn them aside. So now our house is lip-kissers, too. (But only amongst the 4 of us).