I did this once... a long time ago.
I was living with this guy. He was recently divorced, I was young and dumb. I'd been living with him for 3 months.
I hadn't seen Doug (ex) in 1 1/2 years... didn't really think about him - I was OBSESSED with Scott (man I was living with).
So, one morning I wake up, he's up on his elbow, watching me sleep and smiling... I couldn't remember his name! I really tried and finally guessed, "Doug?" There went the smile. And the next week too.
The relationship didn't last (on and off for 4 years - what a waste!)- he didn't trust me before that (ex wife issues) and he never did after that either.
I'm married to a man named Scott now... he tells me that whenever I'm mad at him, he figures I must mean my ex.