I've gotten them twice...
Once we got stuck on Splash Mountain and had to be escorted out. We were given a yellow slip of paper. It was good for a bunch of things, but I don't remember the details. The CM's basically said, "Come back to Splash Mountain later if you want, or use them for something else..."
Once we had really terrible service at the Crystal Palace. Like ridiculously poor...I don't remember the details, but I think we were there some unbelievable amount of time, well over an hour and a half, and only saw one character, and I think they all left for the parade...something really unusual. I always love the character meals. I wrote something on the receipt about being disappointed (it wasn't the server's fault). We were on our way out, and a manager came over and gave us a yellow slip good as a fast pass.
I think it's a pretty smart customer service move for situations like that.
Once we also got that special fast pass that was like 6 fast passes at Hollywood Studios during the year of a million dreams (or whatever that was called). That was much better, but something totally different. I wish they still did that!