Can't speak for a cost, but I thought this one was funny...
A company I used to work for used Lotus Notes. For some reason, and I don't remember exactly what the reason was, I set up my e-mail to copy my mail to another account. I think it was just a "hey, I can do this" thing, playing with the e-mail system. Unfortunately, I made a typo in the name of the account to forward to.
When I came in the next morning, the e-mail system was running really slowly. Everyone was complaining about it. I logged into my e-mail and, low-and-behold, there's all sorts of e-mails in my account complaining about how it couldn't send this message to the other account and, of course, the contents of the e-mail was a message that it couldn't send this message to the other account, and the contents of that message was a complaint that...you get the idea.
I turned off the script and deleted all the e-mails. And, suddenly, from the office next door, I hear, "Hey! E-mail is working again!"
Shhhhh...