There was a pretty notorious thread on Something Awful back in 2006 that started when a guy from Florida posted the story that his father had broken the news that he had had a child with another woman that he'd never told his family about - a daughter about the same age as the guy.
Arrangements were made for her to be introduced to the family at a Thanksgiving Dinner and as soon as she came through the door the dude recognised her as
Kacey, then a pretty high profile porn star... who he'd jerked off to multiple times.
The original thread is locked but you can
read the initial post here. No proof either way, but its a good story.
Not surprisingly, he reported that "dinner was SUPER awkward"...