This will make little difference to the vast majority of folks who read this blog, but we like to note changes like this as publicly as we can. We've removed the Full Page and Details View for anonymous Pens.
It was kind of weird to have Details View at all for anonymous pens, since there was a comment thread in which nobody would be notified and hearting in which nobody would ever see (including yourself). We just 404 these instead and offer no navigation that could get you to there.
We also removed the Full Page view for anonymous pens. This is an abuse-prevention step. Anonymous pens (obviously) have no owner and thus no liability for what is created and shared. If a bad guy makes a malicious Pen, they will almost certainly share the Full Page view with as little CodePen UI as possible.
Now we just show this warning instead, explaining the situation and what to do:
Having an owner for all Full Page View Pens means that it is easier to stop abuse and make it just a little harder for bad guys to do their thing.
For good guys, it's still very easy to get to Full Page View, just sign in and save it or fork it. That way it is now owned by you and you can get to the Full Page View. If you don't have an account, creating one is free and just takes a second.
Plus, after you do log in, you get all the other cool benefits of CodePen. Like the Details View which is good for sharing and conversation, Debug View which is great for testing, Full Page View for quick testing and jumping over to BrowserStack, a personal profile, and all that good stuff, for free.
Onward and upward!