"Sorry, Something went wrong, we are working on getting this fixed as soon as we can"
An error when you want to switch over to a classic page from the new page experience switching panel.
I've encountered this problem where I accidentally wanted to switch to a classic page from the new page experience switching panel and it broke facebook completely in my browser - even rebooting didn't help. Only clearing the cookies for facebook and re-logging in.
Or maybe it's not even a classic page but I just don't own it?
I reported the bug so maybe it will get fixed.