But your cookies are turned on, you’ve cleared them along with the rest of your cache, and the problem exists in every browser!

A typical scenario is when you’ve copied your website from it’s test URL to production, or maybe you’re even just syncing down your prod data to your dev environment, as I do every once in a while to make sure I’m writing code against realistic data sets. After restoring the copied site files and importing your MySQL database dumb into your dev db, you innocently try to navigate to your /wp-admin page and log in. And you can’t log in because of some weird errors like “Cookies are blocked” on your system … frustrating.