After trying out the new features in the 1.1 update, I opened up one of my creative worlds to make edits. When I clicked on it, the screen first read "Syncing pack to server..." followed by a green loading bar reading "Generating world; Building terrain". The bar would load from about 30% to 90%, then the game would quickly close. The world was meant to serve as a wide open review map for texture packs and contained the majority of items, blocks, and passive mobs. I last updated it in 1.0.5 to add command blocks, and found that it was still viewable, in 1.0.9, but it no longer opens.