I once wiped my Hard-disk.
By executing rm -rf / (I hit enter before specifying the directory/file) I was Linux Noob back then, & was literally in tears for weeks after the incident because I didn't backup the Linux installation with my files). I have learnt a very important lesson after that day!
Tldr: fucked my Ubuntu System by executing rm -rf / command and was resenting the decision for weeks to come.
*Edited typos.