My own brain, with its short term memory limitations and tendency to procrastinate. I closed a personal bank account over three months ago, taking the proceeds as a cashier's check. I actually forgot about the check for about two months afterwards (had a baby, dealt with holidays). Then I procrastinated until yesterday on opening a new account at a different bank, planning to use the check to kick off the account.
In the category of "adults should know this" and a PSA to those of you who don't, cashier's checks expire. It's printed clearly right on the check. In particular, mine expired 90 days after issued. 90 days was two weeks ago. D'oh!
Fortunately, they can also be reissued from the original bank quite easily, but man, that was a heart stopper thinking I had just lost a significant chunk of my saving because I was lazy and careless.