A book I just bought a month ago, but I can't find it now and I'm about to buy it again.
(ok maybe not the most difficult, but one thats pissing me off trying to find the past 2 days)
Other ones I can think of, having to pay for 2 different health insurance because the USA university wouldn't accept my other one which is more comprehensive. Buyers remorse from my first car, which wasn't even a nice car.