Most intense day for me was at the very start of my career. Internship... went with product manager to client's office while PM installed new test version of our product for on-site integration testing. Shortly after deploy, client manager came over to ask why production had gone down...

Turns out that manually typing DB name as part of deployment script is not, erm, risk free. PM entered production DB name and took out a very busy call centre for a few hours. Agents in tears, customers raging on phones, etc! After we restored and got everything back up and running, he reached me the keyboard and said "You're doing it this time."

My attempt was problem free, thankfully. Earned many brownie points that day.