Way back in the early '90s when my work was just getting email, a friend was taking the training course. One of the smart alecks in the class was supposed to send an email just to him and wrote something not-so-vaguely suggestive.
Turns out, he sent it company wide! Because the first thing in the address dropdown was the one that went company wide.
My friend, being the quick thinking man he was, quickly hit 'reply all' and said something like, 'Hey, John. Thanks for the compliment. How's that email training going?' Completely diffused the situation and no one got in trouble.
It took years before the finally changed the first listing in the dropdown to go to the IT helpdesk, instead of company wide. It took another few, more innocuous mistakes before it happened.