In the "1 new item in your Stack Exchange inbox" email, there is an "Edit email settings" link at the bottom. If I am not logged in to stack exchange, this goes to a "Page not found" page.
I found this related question, but the answer there says this was fixed in May but I just clicked on the link in an email I received today and still got the "Page not found" page.
Usually, to comply with CAN-SPAM, unsubscribe links don't require a login. If a login is required, you should at least prompt me to log in rather than giving an error page.
Here is a screenshot of the email I am referring to. I'll note that there is no "unsubscribe" link, only an "email settings" link.