I think the accidents occur when people don't affix them properly. From memory they come with a big clamp that grabs on to the architrave around door frames - if your door frames are narrow it might slip off. DH got around this issue by affixing a great big u-bolt into a supporting beam in our house and attached the jolly jumped to that.

Also, my DS's osteopath said they are safe from a joint/development point of view so long as you don't leave the kid in it for hours on end.....