Horror stories:

Associated Press has a story January 11th 2009 about an "Reply-all e-mail storm" at the State Departement. An e-mail was sent out to hundreds of people with all addresses in the To-field and the whole e-mail system was nearly knocked out because of people clicking Reply to All, to either ask to be removed from the list or tell everyone else how stupid they were for using Reply to all...
Read more: Reply-all e-mail storm hits State Department

Daniel tells us: "A person sent a mail to all his contacts when he quit his job, to tell everyone where he was going etc. One of his recipements was a hardware seller who instantly used all the addresses to advertise their products. That wasn't popular at all."

Bad examples:

"Thank you for purchasing the Intimate device XXX, we also have a new great device called..."
The customers definitely wouldn't want others to know that they bought such devices and never agreed on having their address sent out in this way.

"You are receiving this because you went on the 'Save your marriage'-course, we now have a new course called...".
You might have people who know each other and really doesn't want the other to know that they are having problems.

"This is sent to everyone who still hasn't finished the course 'Advanced databases', there will be a new exam..."
What if one of the people on the list is a colleague of someone who doesn't want people to know that he failed the course?


Also see the example on the How to Bcc page
If you want examples on the correct way of using Bcc, look here.