@danonymous,
I was dating one guy for a bit -- and previous to that I had dated someone else...in error once I called my current bf the ex's name -- we both just laughed. He knew I had no interest in my ex. I think it can be an honest mistake -- habit. If you feel confident in your relationship then you have no reason to worry.
As an aside -- I then dated this man for 7 or so years so the mistake didn't hurt at all -- he did end up being an ex- and I made sure I wouldn't make that mistake again so the next one (and ended up being hubby) shared the same name ..... really just purely a coincedence....really.