As an intern, I had a manager tell me that each developer will make at least one giant mistake during their career. The important thing is to have a plan of action, be transparent, and ensure that a similar mistake won't be made again.
Every now and then a story gains some traction that deals with an employee mistake, whether a small typo or a system design issue that wreaks havoc. Most recently, the story of a fresh-faced developer that dropped production while setting up his dev environment [0].
I'm sure there are many more stories that would could inform others what not to do or what warning signs to look for. What was your biggest mistake, how did you remedy the situation, and what were the repercussions?
[0]: http://uk.businessinsider.com/worst-first-day-ever-reddit-2017-6