I may have told this story before. But many moons ago I was working for a largish international company (offices in NZ, Australia, the UK and Canada). An engineering intern had been allowed near the internet and read about the latest computer virus spreading around the internet.
Armed with this dangerous knowledge the intern decided he needed to warn everyone in the company. Instead of using some of the distribution lists (the memory is lacking as to whether or not he had permissions), he added every single person in the company to the To field.
This made the email about 2MB in size with all names added. Which meant the mail servers got hit with about 2GB of emails. Which they did cope with.....until a number of helpful folk hit Reply All. After which the mail servers expelled excrement and fell over taking most of the network down as they tried to handle the load.