I remember once on a complex project a senior manager going "That may be what I asked for, but its not what I want" and in a startup I joined where the CEO was adamant that any feature he could think of could be implemented in less than 48 hours - and this was a video technology/devices startup not a web app!

I've made horrific technical mistakes at various points in my career - but at least it is possible to learn straightforward lessons from those. Learning about people - now that's a challenge!