It's easy to do stupid things while drunk, but it takes a special kind of stupid to do it sober. Also it serves as an alternative to the drunk thread for teetotallers or those who don't need alcohol to exhibit extreme bad judgement.
The stupidest thing I've done while sober is probably uttering the line "How hard can it be?" this may not seem stupid to you, but the sheer number of times that has led to me doing something INCREDIBLY stupid/painful/embarrassing e.g. going down a hill in a laundry basket, attempting to tackle the biggest guy in school on my own during bulldog (should my foot bend like that?), going over a one of those crazy bridges in a wheelchair (and getting launched out of the wheelchair) makes it the stupidest thing I’ve ever done.
I still do it, which proves I’ve failed to learn my lesson.
(In fairness it’s lead to some cool stuff as well… but mostly it just leads to pain and embarrassment)