With the latest Lightroom CC, in Develop mode, doing Spot Removal, Graduated Filter or Adjustment Brush, after making some changes going into Full Screen Preview (F) locks up LR. No action will exit Full Screen mode. I have to Cmd-Tab to another app and then force a quit.
See this bug report in the official Adobe feedback forum for ways to avoid triggering the problem: LIGHTROOM 6 - problem when viewing images and video full screen (F key related). Please add your vote and opinion to make it more likely Adobe will prioritize a fix. If you reply or follow that report, you'll be notified of any fixes and workarounds.