I'm not sure what's at fault - Yosemite or Minecraft.. And I'm not sure if Minecraft is a red herring. I have Yosemite on our Macbook Pro and on a new iMac. On the macbook, after someone has played Minecraft and the computer is re-opened from sleep, instead of getting the login we get a black screen with spinning beachball of death. It never clears up (even after waiting an hour) and no key-combo will bring up any force-quit menu. We have to hold down the power button to reboot. On the iMac, I noticed a similar-ish thing: My son was playing Minecraft (full screen), which went fine except when he pressed the "Quit" menu option on the screen, the screen went black and all we had was the arrow. NOTHING I could do would bring back the desktop or Finder. I tried the ESC button. I tried option-command-ESC, but that wouldn't bring up any force-quit menu. I tried clicking in all four corners. I tried waiting an hour. Nothing. Thoughts? And more generally: why would a modern OS like Yosemite be capable of getting hosed by ANY app? I never see iOS lock up fully after using an app. I only see individual apps crash. Come to think of it, Windows 7 never crashes like this for me either (at work).