In middle school, halfway through the school day, I kinda just packed up my backpack and walked out of school to go to the buses. I freaked out when the buses weren't there. Then I realised that it was only halfway through the day and tried to get back in the school. Of course, all of the doors were locked and I spent a good 10min trying to get back in. Luckily, there was a class in the gym and a couple minutes of pounding on the gym doors got me back in. If not for that, I probably would have been out there the rest of the school day.

similar:
unknowingly deleted all school accounts, made a backup of the accounts list (empty file)overwriting the old backup.

fixed it by manually recovering the old backup from the hard drive, not an easy thing to do on an ext4 partition...

Once was doing testing on a robot with an arm on it that could only be described as a scythe. I wrote up it's C program, transferred it to the robot, compiled and didn't move back before pressing the "run 'rohmba_test.c'" button (the original programmer misspelled roomba.). I heard the "zeeet!" of the arm servo beginning operation and next thing I know, "WHHAAAACK!", oh the sensation of a metal robot arm to the head in the evening!

I sleep in the social studies class... Suddenly my teacher call my name and I wake up like a vampire... The whole class lol

Trying to install Arch GNU/Linux. I thought I was hardcore so I skipped Ubuntu and installing it in a virtual machine. Screwed up while partitioning, a Windows 7 gone.
...And I didn't make the recovery disk (laptop) so I had to buy another one for $50.
Luckily, I backed up daily automatically.

Oh, and I'm all for the Off Topic forums, I just don't think the topics should come in the "most recent topics" on the forums homepage. Don't know if that's possible, though.

GNU/Linux is actually quite nice once you get used to it. Which version of Ubuntu did you use? 10.04 is a lot better than previous versions.
Oh, and I did set up Arch eventually.