My brother forgot to clamp the ladder on my van once and it flew off the top of the van whilst I was doing about 65mph on the M5. God knows how it didn't result in a pile up but thankfully it didn't. As the ladder flew off it was clipped by a lorry following us which catapulted the ladder into a field...very lucky indeed
Very lucky doesnt even cover it! Could have been taken to court and fined quite heavily for that one, classes as unsafe load. Extremely lucky! Right place and all.
Ive NEVER had this happen to me. Mind you being wfp my collapsible ladders are in the back now

I always made a point of putting the ladders clamps on my drivers chair, without fail. This way if i put the ladders on and forgot to tie on straight away, when i went to get in and felt a hook prodding me up my ar*e it was a timely reminder!!
I have put flasks, lunch boxes, squeege and applicator (never found these), and a phone (wedged onto spoiler 1 mile down the road when i remembered!!