I had located a spawner and dug to it.
As soon as I reached the spawner and made it spawn a zombie, the world crashed, automatically closed the game, and the launcher gave me the bug report.
Bug was created by finding a spawner in a desert and making a mob spawn from it.
Environment
OS: Windows 7 (ver 6.1, arch x86)
Java: 1.7.0_21 (by Oracle Corporation)
Launcher: 0.8.1 (Dev) (bootstrap 2)
Please attach the crash report, otherwise this ticket is incomplete.