I fell in my first year. A hot sweaty day, tired and in a hurry!
I had a few scrapes, and a couple of bruises; some nasty; but nothing serious. I landed with my legs through the ladder, sort of standing up.
I think that unless you aren't human, the chances are you'll make a mistake with a ladder one day.
The laws of averages suggests that if you use ladders on a daily basis; say climb a ladder 50 times per day (which is a tame figure), four days a week (just say), that's 200 climbs a week. So imagine you work a slack 40 weeks per year, that's 8000 ladder climbs per year.
Imagine you work 10 years as a window cleaner, that's 80,000 ladder climbs, and my figures are based on really low estimates; it could be a lot lot more climbs than that.
The chances are that during one of those 80,000 climbs, something bad is going to happen.