I used to be a contractor PM for a small outsourcing company.
One sunny day, I was discussing with my colleague, a branch head, some unpleasant issues about how we optimize our expenses, shifting people, etc. Not a cheating, not a fraud, but clearly something that "should not go out these walls".
It was an important discussion, and I wanted to CC to a top-level manager to get his approval on our further actions.
Accidentally, I have placed a name of a customer instead of my boss' name and sent the message.
Luckily, it has nothing to do with that particular customer and his project that I led, but still, I have never recovered after that story. Everyone, including the top manager, told me, "don't worry, it happens to everyone", but at the end of the day I have asked to resign from the customer's project and never got further projects with that company.
That was, indeed, my biggest mistake for all my career. I was doubtful for quite a while if I could ask this question here, but now I think I can.
I'm thinking I could have recovered from that situation.
How to correct myself after sending a sensitive information to a wrong person?
Update, to be most precise.
(1) The cost of actions the company has to do (staff and workflow re-arrangements) was quite comparable to entire profit I have earned for the company (I've been working just for several months).
(2) Yes, I'm a kind of maximalist by character, and such failure is not compatible with my views on the business. If I were my boss, I would do the same: say, "it happens to everyone", and avoid assigning further projects to that person.
(3) By "to recover" I understand an attempt to restore trust by the customer and colleagues. Other aspects are out of scope of this site.