As someone who uses ladders very occasionally and treats them with all the respect I can muster, can anyone tell me what actual mistake did the guy in the vid make?
Maybe someone should have been footing the ladder but trads don't foot ladders. How could he have prevented the accident without footing it?
The man made two basic mistakes: firstly he didn't check the ladder footing to make sure the feet had firm ground under them to prevent slipping (if he had he would either have not set the ladder or would have taken some precaution to prevent it slipping); secondly he set the ladder with the styles in contact with the railing so when he climbed the flex of the ladder caused it to pivot at the point of contact and lifted the feet enough to start the slide.
As someone pointed out earlier, he should have attached the ladder to the railing - a bungee would have been ideal.
Yet again it proves the point: ladders are not dangerous, it's the user that causes the accident.