It happened to me in the park one afternoon and it scared the crap out of me! My girls wear the step-in harnesses and I've never had a problem until that day...I'm still not quite sure what heppened but Megan (Moo-Moo) was behind me and I felt a tug on her leash. I looked back and she was just completing some kind of backwards summersault and she backed right out of her harness
I almost yelled but knew I would scare her so I very happily said to my husband "Oh look...Moo-Moo is running loose in the park! Come to Mommy Miss Moo!) and it worked...whew! She came right up to me, I grabbed her up and we got in the car to adjust her harness before going back outside. After that I bought the vest harnesses...they seem a little more secure but I am still careful because we travel with our girls and i have to know htey are secure when we're out of the truck.