Occasionally, I've found that the 'Loading Project' dialog doesn't go away, ever. Even worse, it floats above all other windows (right now, I'm having to move it to see this text in my web browser.) It says "Synchronizing Local VCS".
I can bring up the 'File' menu in IDEA, but selecting 'Exit' doesn' do anything. Command-Q just beeps. The red 'close' box on the dialog doesn't do anything. Force Quit is the only way I've found to get around this.
If I force IDEA to quit, and then bring it up again, things work fine.
I've seen this happen with IDEA 700 & 811.