I have in fact, experienced a roll back. On the rare occurance on KK I was a lucky one. I will explain how:
I was at the park errmmm...late August and it was clear when we went on the queue for the ride and after about an hour it started to drizzle, and the ride closed for 10 min, no big deal, no serious talks of leaving. About 30 min later queing for the front row it starts POURING. After 15 min, the talks of leaving came up and I was very very pissed. I made them stay and we happen to be the first train of the first launch after the rain. The track was still wet and caused the train to be slower, ok well onto the roll back...
Normal launch, up the tower, getting the top and cresting the top(in the front) and seeming like we're going to make it over but it kept going slower and slower until, BACKWARDS! Amazing feeling freefalling backwards and a twist back to the right.
It took a good 5 min to get back to the launch position and the next ride made it over.
Rollbacks are fun, like Robbeal said like a drop tower but the sweetness of it, is two consecutive launches.