When I was young, I learned to use my dad's PC the hard way. "What's this directory called DOS? I never use it. <delete>"
I also recall recovering a CRT screen with a faulty on/off button. I fixed it...but learned later that those screens have liquids in them that could explode when handled wrongfully (and yes...you could say that was EXACTLY what I did).
Installing windows millennium should have warned me, but I still went and installed windows 8. It was reasonable (though kind of retarded that it works best if you hide/remove everything that makes it windows 8)...but it got screwed up on windows 8.1 again (I admit...pirated version).
One of the biggest screwups at work was my fault, though I still blame stress. Amongst one of the few dozens of mails that arrive on a daily basis (and have to be handled "NOWWWW!!!!") was one from human resources that had a "user out" excel sheet file in it. Which is exactly the same file as our "user in", but with an included extra date right below. So I opened the file, saw that <random guy somewhere in another part of the country> had left the company about a month earlier, shrugged to myself as our HR department rarely (if every) delivers these notes on time, and deleted the user.
Later that day, I got a call from the supervisor of said guy. he couldn't access his account anymore. A bit puzzled, I told him I removed the account on request of HR. At the same time, I bring up the file. And while on the phone, I realise the error: I had misread the user in date for the user out. The poor fellow wasn't to leave the office for two more months, so he still needed that account. And quickly, as he had a night shift coming up.
Worst part of it was that granting permissions, recovering rights and all that jazz takes a whole lot of research. I spent practically the rest of the day making sure that everything was okay again and retrieving his mails. Him and his superior understood my situation and were happy I did everything I could to solve it, but man, I felt bad about that situation.