"Reply All" shuts down German Bundestag Mail (Google translate)
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http://blogs.technet.com/b/exchange/archive/2004/04/08/10962...
tl;dr - Bedlam DL3, a famous "Me too" reply-all incident at Microsoft in 2004, on the order of 15 million total messages.
I kid, I kid...
Is it bad though? Should we really expect people to process all emails at once and only then act on them?
In this case, "Should we really expect..." is like saying "should we really expect people to read a dozen emails before sending 5000?" Yea, I think so.
One of our consultants wrote off a sarcastic reply-all email in response detailing what the reply all button actually did as if explaining to 5 year olds. The client demanded that this person be fired.
This may be too charitable, though.