I've just unsubscribed myself from a mailing list and received this delightfully poetic, if plaintive, acknowledgement in response:
"We have removed your email address from our list.
We're sorry to see you go.
Was this a mistake? Did you forward one of our emails to a friend, and they clicked the unsubscribe link not realizing they were in fact unsubscribing you from this list? If this was a mistake, you can re-subscribe at: [redacted]"Hopefully everyone already knows to edit out any "view this email online" or "unsubscribe" link before sending it on (I have to do this every time I forward any mailing list email to my Posterous blog for example)... but that's why. These emails are packed full of live links that include your email address and can be used to unsubscribe you :)
Perhaps it would be better to have this sort of thing as a two-step process - click to unsubscribe, but nothing actually happens until you confirm.
And while I'm at it, don't forget that if you produce a mailing list email newsletter thing which includes an unsubscribe link you also need to (as in I think you should, it's not a requirement) include a ilink.
This is because (do I really need to write this?!) you might hope that people WILL forward your email and the recipients might subscribe.
I never trust those "forward this to a friend" systems that require me to enter my email address and someone else's (I think it's rude to give you my friend's address without checking with them first which is why I always use my own email system to alert them to something they might want to join). Does anyone actually use them?