Game crashed when I "mined" a jukebox in my house. When I logged back in a large portion of my house near where the jukebox was had disappeared and a pool of water had appeared (see screenshot).

I had just updated Minecraft on my PC from 1.3.2 to 1.4.6, used MCPatcher to patch the .jar file (to use HD texture pack) and pasted a save file from my laptop in the saves folder. Not sure if this is relevant but thought I'd mention it.