I was lucky - I spotted it while I was cleaning my bike. Imagine what could have happened if the crank had broken when I'd been standing up and sprinting, or pushing down on it on a fast downhill bend ...
I did snap a crank once but it happened when I was sitting down and riding in a straight line so I wobbled a bit, but came safely to a halt. I then had to ride 5 miles home one-legged, with the crank in my jersey pocket so I could take my pedal off when I got back!