Biggest whopsie as a 15 year old building servers was starting a server with case covers off to check whether it boots, then dropping a screw on the exposed hard drive board. It left a neat trail of sparks and smoke as it bounced down. 200mb full drive of insurance company data burned in front of my boss watching.
Later in career, different place different times, my colleague and best friend ran a mistyped grep on prod server that tried to suck up all of the files content redirected into a single file. AIX in my limited experience is an unbelievably robust system - but it does Not handle root disk out of space well. Or at all.