Yes, this has happened to me before! Very scary.
I don't have the time/patience to replicate it right now and take a picture to show you, but I saw that the second time it happened, it was because Bowpi's new leash worked with her old collar and the new tags we had on her like a perfect puzzle. You know those little metal puzzle toys where everything's all twisted into a knotted mess, but if you maneuver things just perfectly, it all slips apart? It was like that.
A little complicated to explain… basically, the spring-loaded pull-back clasp of her leash would sometimes fortuitously catch onto the O-ring that connected her tags to her collar, and with a gentle little twist, she would be able to release herself from the leash, just like that.
It was kind of amazing, and I'm glad I saw it because it remained in position for only about a second before she slipped loose.
Needless to say, we got her a new collar right away and twisted it up in all sorts of combinations to make sure it wouldn't snag the same way. We haven't had the problem since.
FWIW, we use Red Dingo leashes with the classic "Old Faithful" clip:
http://www.reddingo.net/dogbiteclips.htm
It's kind of neat because the clips are designed to be interchangeable, and they have some other cute designs. Though they're swappable, it's sturdy; the clip itself is in no danger of coming apart.