I happened across an interesting situation with a client a few days ago. The client was finding subscribers to their newsletter were contacting them to say they weren't receiving a newsletter anymore.
After much head scratching, the client realised what was happening:
The subscriber had received the newsletter, and liked it so much they forwarded it to a friend. Instead of using the forward link in the email they received, they just did what anyone would do and press the forward button in their email client.
Well, the email contained a link that allowed the subscriber to manage their email settings. The link was specifically for the subscriber so they didn't have to worry about signing in. When the subscriber forwarded the email to their friend, their friend hit the manage subscription button, and updated the details to their own.
If you do have an unsubscribe / manage link in your email it needs to be protected from change, either by confirming the changes via an email confirmation message, or requiring the subscriber to login to manage their details.
Check your own newsletter system to see how unsubscribes and newsletter management is handled. If it doesn't protect your subscribers information from being changed or removed without their consent, bring it to the attention of your newsletter system provider, or change to another provider.