Losing all your exam certificates.
When you get these bits of paper, they are so precious. But the years go by, you move house, you move country, twice, you have had a number of jobs, these things become completely unimportant. Or so you think...suddenly, you are required to produce them....and they are nowhere, nowhere to be found, every box, your attic, your mother's garage, covered in dust, mice, grime....you find everything from old wedding invites to clumps of the Berlin Wall, but not you what you are looking for.
The process of getting replacements is a bureaucratic nightmare....particularly if they can't find you because your surname changed in your youth and you can't remember under which name you actually took the damn things.
The moral is, children, take very good care of all your paperwork. Always.