So I've tried to unsubscribe myself from a newsletter for the past two weeks ...

It's a service I used exactly once and apparently I got signed up for their newsletter somehow.

When I got the first mail two weeks ago, I clicked that unsubscribe button in the footer to get to their unsubscribe landingpage and hit that confirmation button.

It said something like "... within 24 hours ..." and I was okay with that, but I kept getting a newsletter / offer / whatever every single day for the following two weeks.

Today I figured out, why the unsubscription seemingly had no effect.

I used a GMail alias for that website, so my address contained a + character.
Apparently they just put that mail address into some unsubscribe link template without urlencoding it, so that + turned into a space and they attempted to unsubscribe some mail address with a space instead of a + ...

Remember kids, always urlencode stuff in URLs!