When I first started using Outlook, it had a "nickname" field (it still does, but you need to add it manually to a contact). Partly as a joke, and partly messing around to learn Outlook, I gave a few friends of mine nicknames. I sent an email to one about something totally random. Next time I saw him, he asked why I'd addressed his email to "fatboy". I didn't realise that Outlook used the nickname, and labelled the send to field as <first name>"fatboy"<Surname>, with <first name> and <Surname> being his actual first and surname..
Then there was the time I sent a "Happy Christmas" email to all students at my Uni. I left read receipts turned on, got roughly 18,000 read receipts, which blocked my mailbox, and earned me a telling off from the technicians, and a surprising amount of email abuse from those students who didn't celebrate Christmas for whatever reason, as well as those who did celebrate Christmas, but got offended on behalf of those who didn't. All I wanted to do was wish everyone well.