I know what you mean all too well.
A few years ago I was on the bus, saw a guy younger than me playing a DS and asked what he was playing because my brain shut off and decided to be friendly for once. It led to a conversation with one major major problem, I assume this was the only social interaction this guy ever had. Only ever talked about Final Fantasy. I was going to the net cafe where my friends worked to hang out and he just...followed me, and tried to talk to them. It took a good hour before he got the message and left.
My friends just looked at me and asked what the hell is wrong with me, I bought a round for us all when we went to the pub to make up for it.