It might be related to osx and powersaving/putting the process to sleep (by minimizing the window).
I also believe there is a difference in being disconnected and being dropped from the game ? Might be hard to reproduce if you can't sleep the process as I suppose OSX does when it's minimized ...
I have a headless server to be able to switch locations without bringing my saves with me all the time. And quite often I find that I have been playing only on my local save without being connected to the server. I did some research and found out how my playing pattern is causing this.