Your 'oh shit, what have I done? 😳' moment.

Forgot the non-written-down password for three full disk encrypted critical application VM's AFTER I shut them down

Make a change to the ssh config and then type exit. Always a crap shoot if you are gonna get back in.

I used a get request to process my checkout form. Basically you could change the URL with &total_to_pay=0 and my site would send that value to Paypal and you would get stuff for free. It was catastrophic, but now it makes me smile, silly me from the past.