Game crashes and then deletes game file

In multiple worlds throughout the day (7/9/14), Minecraft suddenly crashed, bringing up a crash report page. I tried restarting from the page, from the launcher, and after restarting my computer, but the problem continued. After multiple crashes, my world file actually disappeared (this happened with two separate worlds).

What I expected to happen was...:
The game not to crash... you know, ideally.

What actually happened was...:
The game suddenly crashed in the middle of play.

Steps to Reproduce:
I could not find any common factors that caused the game to crash. (That is, it didn't occur exclusively when I was mining, smelting, in a particular location, etc.)