Actually, I find Ancestry to be hilarious sometimes. I was looking for someone and I came upon a man called James and his relationship to the head of household was given as "sister"? So I took a look and he was a "visitor".
And, yes I have found one person (a child) who was on the census twice but in different places. Once as a visitor (grandparents house) and again as a daughter. I guess her parents included her as being part of the household even when she wasn't there.