Many years ago a colleague of mine replied to a company-wide email from the IT department which contained information about some new system or other being introduced.
My colleague knew the member of the IT department and often enjoyed robust banter with him, so he replied to the email with some jolly, and I have to say, expletive-rich banter...
...unfortunately for him, he clicked on 'reply to all' instead of just 'reply', which sent his email to about 2,500 other company employees simultaneously.
(He kept his job, but not his reputation!
)