The worst one was quite a few years ago when I still had all of my computers up and running properly and my personal Linux file server still worked.
It had a to separate drives in it, a small one for the OS and a big one, 120GB at the time I think for storage. Something had happened to the OS drive and it had to be replaced, out of habit I copied all the stuff off of the storage drive on to my machine while I swapped the drives around.
I made the mistake of storing all the stuff on my server, my music, all the patches for my games, pictures, documents, videos, in a single folder on the desktop. Without a single thought for what I was doing at about 2:30 in the morning I selected the folder named "Server" and Shift+Delete'd it. Shift+Delete for those that don't know, bypasses the recycle bin and 'permanently' deletes the file. Of course for me it was permanent because it was a time before I'd heard of such things as hard drive recovery tools.