I'm not sure what you mean by "What do you think?" It happens. I've known a child that broke his femur (thigh bone) going down the slide with a babysitter, and was in a full leg cast for upwards of 6 weeks. And if you think about it, it makes perfect sense. There little bones are meant to support their low weights, not their own weight plus the weight of an adult. Not to mention the speed that can be achieved going down some of these slides... Either let them go down the slide themselves with you standing on the side, or don't. Why risk it?
ETA: I agree that accidents happen, its part of growing up and how you learn. My own kid broke his arm falling off a balance beam at school. But I don't have any bad feelings about it, because it wasn't anything I did or anyone else did that caused it. I'm sure there are all kinds of neck and back injuries from kids falling off the monkey bars upside down, and I cringe everytime I see mine do it, but what's more fun to a kid than hanging upside down? :) There are risks involved with any activity, but if I can minimize the risks that I create, without increasing the risk to my children (for injury or over-sheltered-ness...is that even a word), you better believe that I'm going to!! You have to find your comfort level and go from there, everyone is different and that's the way it is supposed to be.