I find it funny, too. Once you meet someone, you cannot "un-meet" him/her. There is always a connection, - acquaintance, familiar face, friendship, love, hate...
but not oblivion.
And people tend to come back, no matter how FINAL final good-bye was.
I just had it the other day - someone from the past, someone I thought I would never see again. I did not remember his face, his name... and he jumps at me out of nowhere "Where have you been? How have you been?"
I did not want to say that not only I don't remember your name and you, I am pretty sure you don't remember mine. So I started with my usual non-sense "I have been here most of the time, except for the trip to the Moon." They guy did not seem to get it, but his friend whom I saw for the first and (last?) time in my life was laughing.
I guess I said "Good-bye" again. Will it happen again? Deja vu? "Hi, where have you been?...."