Never had it happen, but you better believe I am conscious of the fact that such a slip could occur.
For the first time in a while I have a GF/Mommy and as things are getting more and more serious, I have told my Mother about her. Whenever I talk to my Mother about my GF, in the back of my head I am always on guard so that the word Mommy doesn't escape my lips.
I wouldn't be surprised if this is something that all ABs and Littles have trouble with. I mean, at the end of the day, when you have a partner who is also a caregiver, those titles; boyfriend, girlfriend, Mommy, Daddy, they all sort of blur together in our AB/Little world. My GF is my Mommy, the titles both stem from a place of affection and in all honesty, my current partner is both of those things. Furthermore, when you're an AB and you have the acceptance and comfort of a knowing partner, it becomes a standard reaction to simply call them Mommy or Daddy. Calling a GF or a BF Mommy or Daddy in our world is completely normal, that is where the danger of a slip lies. Calling them Mommy or Daddy becomes so normal that when we enter a more traditional social circle, we risk forgetting the fact that regular folks don't have a BF or a GF who is also a Daddy or a Mommy.
So yeah, it's something that I have thought about and been worried about, but I have yet to let it slip accidentally when talking to friends or family. Like you, I am curious if anyone has accidentally let that slip and what the reactions were.