Upon trying to load a world, it crashed trying to render a falling sand entity, which I believe is set to a block ID of stone. This world worked fine in 13w38a as I was playing with the secret settings. This has happened twice with the same result.
Attached is the crash report and world folder (zipped under "FallingSandCrash").

After posting this ticket, I removed the affected spawner, and the world works fine.