I accidentally triggered one of these many years ago. Somebody sent a CraigShergold-gram to the building list (about 5000 people) one Friday afternoon, and I thought for a moment about whether I should send a "Please don't reply, especially to the entire list, here's the explanation" reply on not. This was back in the days when most of our mail was on departmental Vaxes, and some of it was on a big Unix-like mainframe system, some running SMTP but some running UUCP, and there wasn't a snopes.com to point people to, just alt.folklore.urban on Usenet.
I decided that it was probably better to send it than not, and of course I started getting bouncegrams from people on vacation, etc. About half an hour later, a friend called, asking if I'd meant to send five copies of the mail, spaced five minutes apart. "Umm. no..." "Thought not, enjoy the rest of the afternoon cleaning things up." Apparently the mainframe in the basement had forwarded out the message to everybody, decided that something hadn't worked, and re-queued it to try again later. I went down to the basement where the building sysadmins lived, apologized, and we spent a couple of hours trying to find the problem and clean up the mess (simply stopping the mail server and clearing out its queue wasn't close to good enough.) Never did find out what was wrong, and of course my bouncegrams were starting to include "Can't deliver message; mailbox full" as various departmental Vax disks filled up. And Monday morning we started with a couple rounds of "Stop sending me this junk at work" "Get me off this mailing list" "Don't send that to the entire list, dummy, just the sender (sent to the entire list, of course)", but none of them broke the mail relay this time.