Been there, done it, have the t-shirt
These things happen, I have done it 3 times this year after;
1. Having a new newsletter system coded. In tests it worked fine but when I used it for my subscriber list it sent to all my unsubscribes and addressed every email to the first person on the list. Was rather embarrassing.
2. Had the systems redone and tested yet again. This time when I did it for real it emailed as an attachment, the entire subscriber list. To quote one unhappy person "Thanks for letting everyone know my email address, really appreciated"
3. Ticked the merge box. Now I emailed my unsubscribers, my subscribers, and ALL my clients.
A lesson learned........ Test, test and test again. However, sometimes these things happen and even though large companies get held accountable more so than us smaller ones, everyone makes mistakes ;-)
OK - posted anonymous for obvious reasons and please. FLAME ME all you like ;-)