This happened when I was trying to get Blockland into fullscreen mode. It then disappeared, and was inaccessible unless you loaded it again. One other time I attempted this, I hit apply, and then exited from the game. Now, when you load the launcher, it crashes after the game boots up.
Details:
Macbook Pro, bought in 2010
Running on OSX Mountain Lion v10.8.5 (i think)
Crash report is attached to post