Many things and it's usually something very important. That's why I put it some place "Special" and could never find it again. I would've been better off to have left it in the junk drawer with everything else that never goes away. : )
I am the absolute KING of doing that, in addition to setting "super secret" passwords which I promptly forget. A few years ago, I lost contact with an entire group of online friends because I changed an email password that I had been using for about five years. A week after changing it, I forgot it completely, but to make matters worse, I let another month or two go by before I tried to do anything about it.