This happen after the first review on review-beta/late-answers.
After reviewed the first late answer, you're going to the next one. Both button are disabled when you arrive, which is normal but if you edit an then cancel the edit, both buttons are still disabled.
You can reproduce it:
- Go on http://stackoverflow.com/review-beta/late-answers
- Review the answer
- Jump to the next one
- When the page is loaded and before Not Sure button become available, edit the answer
- Now cancel the edit
You're stuck with no available action: both buttons are disabled.
Of course this doesn't happen when you don't cancel the edit.
Here is a screenshot:
As @animuson said in comment, I also have the orange fade-away background sticked.