Setting up a GNU Mailman mailing-list with the 'reply-to-all' setting still set to on (as per the defaults).
It was intended to be a distribution list.
The list had 8,000 recipients.
.
I found out what an ohnosecond is, as the blood chilled a little in my veins.
Turned out not too bad, 6 innocuous replies were sent out to everyone. It was fully open for about 25 minutes before I set 'reply-to-all' to off. Could have been much worse.
Will never ever forget to do that with a mailing-list again.