My "best" rm -rf story was when I created a system user account on a production system (router) to test something. What I didn't know is that for system users their home directory is set to / by default; this will become important later.

I finished what I was doing and wanted to clean up after, so after undoing my changes I ran userdel -r on that user (-r removes its home directory). The command took way longer than expected, at some point I terminated it to investigate as I assumed there was something unrelated going on (high CPU or IO contention).

At this point you probably figured out what happened. By the time I stopped the command the damage was done and a good part of the system has been nuked. Surprisingly (and thankfully I guess) the actual firewalling and routing is done at kernel level so it continued doing its job totally fine despite userspace being nothing more than a smoldering wasteland.