Document Unavailable Error
I had a cloud document open that is shared with others and has co-editing enabled. I am the document owner. I stepped away from my computer for a minute and when I came back I saw the following error:

I received this error once before and selected "Save As..." only for the document not to save and and for my changes to be lost.
What is causing this and how do I prevent it in the future? And is there a better way to restore changes if it manages to Save As... successfully other than copying & pasting those changes in?
