I have been trying to accept a challenge, but each time I click “Accept Game” in the game review screen, it says “Challenge does not exist”.
Then I close that window, reload “Find A Game” page to be sure … and there the challenge still is. I did that a few times, the challenge hung around the whole time.
If this is a challenge that someone else is reviewing at the same time, then it might be better to say so 
It’d also be nice if there was some way that people couldn’t hold up a challenge in review indefinitely…