In my first week in my first paid job...
... I was told by one of the execs to familiarize myself with a PC running THEOS/OASIS that was to be installed the next day at a customer's. I proceeded to test the software by inputting some fake data, generating queries and reports and all that carp.
When I finished doing this, I decided to wipe all the data I created by using an "Initialize" option in the menu. To my indescribable horror, it wiped out all the data, the software and most of the OS.
After twenty minutes of panic, cold sweats and frantic reading of the manuals, I found there was an option to undo the last initialization.
Lesson learned: RTFM before playing with a system you aren't familiar with! 8^)