Can't solve the riddle in the sewer system, has a pressure plate on the ground that activates a question switch to lock you in. Then the Riddler asks "what number comes after three when the Knight falls...the answer is two...one". Then the room floods with gas.
Why don't I strap on my job helmet and squeeze down into a job cannon and fire off into Jobland where jobs grow on little jobbies?! - Charlie
I also died to this a few times until I noted in the death menu that the top right corner said something like: "Riddler has made this puzzle to be unsolvable. It will require ALL of Batman's gadgets to overcome it." paraphrasing of course, but I took that to mean I needed to come back later.
"Behind this mask there is more than flesh; there is an idea, and ideas are bulletproof." -V