This happened at Qualcomm once. Some IT email was sent out to most of the corp and then people's vacation/out-of-office auto-reply-to-all propagated it more. Then people would send reply-all messages like "remove me from this list!" it went on for most of the day.
Also happened at IBM when I worked there. A guy named Armando changed his phone and manage to send a notice to everyone in the company using a maillist. People didn't know about the maillist and started to ask to be removed, turns out it was not possible... It took weeks to solve and everyone at IBM pretty much lost access to email during the period due to the overload. Even worse, at the time a lot of internal systems at IBM were based in Lotus Notes that is a weird framework email client mashup and those systems were also affected.
