Game crashes if you are at the main menu, full screened, and press Alt+Tab

Press ALT+TAB when you're looking at the main menu. If you're in Windowed mode, everything is fine. If you're in Fullscreen mode, the game crashes.

I've never installed a Mod for Minecraft, so that can be ruled out. I'm not sure if this happened in 1.5, but I don't think so.