Oh, i have a brilliant one:
A few years ago i spent a lot of time converting .flac-files into .ogg-files in order to put on my oldschool iPod. As I did a lot of repetitive typing - entering $dir / for file in flac ; do convert etc / mkdir -p $somewhere/$artist/$album / mv $somewhere/.ogg->$new_dir/ and so on - I thought: "hm lets just write a loop over loops for all the artists here and then all the albums and at the same time create the nested directories somewhere else... hm actually in the home directory.... and later love everything on the iPod at once."
so i was in my music folder with the artists-folders i wanted to convert. i did something wrong
So i did my complicated script directly in the shell. I made something wrong and instead of creating a folder "~/artist/album" I created 3 folders in my current working directory: "~", "artist" and "album". hmph dammit gotta try again... but first : i have to clean up these useless folders in the current dir. so i type of course this: "$ rm -r ~ artist album " after about 5 seconds of wondering why it took so long i realized my error. o_O I stopped the running command, but it was (of course) too late and i bricked my current installation. All the half-deleted config files made or impossible to start normally and extremely tedious to repair it by hand, so i reinstalled.