I had to stop payment on a money order once because I lost it. It was in my pocket, I was on my way to pay a guy and when I got there my pockets were empty. I have no idea where it went but I know that it was gone.
I searched around frantically but it didn't seem to be anywhere. It is pretty easy to cash in a money order even if it has not been intended for you so I knew that I had to stop payment immediately or any old person who found it on the street could take the money. Luckily I got it cancelled in time and I was only out a few bucks and a little bit of dignity.