I don't think one password was enough for the MediaDefender employee who used the same password for accounts on pirate TV sites and Gmail, leading to 700MB of highly confidential and embarrassing internal company mail being spread all over the Internet.

And actually, writing passwords down on a piece of paper is not a particularly bad idea, as 99.999% of black-hats won't be able to get access to it. Don't put them in a text file, though.