I always thought that such things were an urban legend of unix/linux environments. "Be careful with your commands! You too could nuke your entire installation!". When I was in college (some 20 years ago) I rolled my eyes every time I heard some old timer wag his finger and recount of sometime some new hire managed to destroy days/weeks/months of work by running rm -rf.

And then, one day, at work I was advising a junior engineer. He were having issues with a software build process that he was tasked with documenting. I explained what went wrong, what he can do to correct for that, and to try again. He said okay and while I stood there, looking over his shoulder, typed rm -rf . and hit enter faster than I could shout "NO!".

I just stood there and laughed for a good 5 minutes before I was able to catch my breath and explained to him why his machine was unresponsive.