[modeless]

You've never seen an email storm then? They're most often started by a mistaken "Reply all". http://en.wikipedia.org/wiki/Email_storm

[pcl]

In my experience, they've been caused by misconfigured listservs.

[afterburner]

I've read hilarious stories about them, but I've never been part of one. (Someone replied to all once at my old job when they shouldn't have. Then they said oops. End of boring story.)