Actually, if you read the original posting on the topic you will see that he was using the ladder to access a single storey roof.
That is THE most dangerous thing you can do with a ladder but because its not very high we are all a bit cocky about it.
Shifting your weight off the ladder onto the roof and then the reverse when you go down are the two times when a ladder is going to kick out or slide.
Two of the most pants filling experiences of my window cleaning life involved flat roofs. The third, was when a ladder blew sideways with me on it due to a sudden gust of wind.
In all three cases I never saw it coming.
I now am almost entireley WFP although I was on my ladder today. The truth is WFP is better, better for me, better for my bank balance and better for the windows. The customers love the results.
I don't understand cleantecross's comment about 4 cleans. My windows are bang on first time although there was a steep learning curve at first.