If you have multiple version of Revit on you machine and you double click a RVT file you might get to see this screen:
I had to read it a couple of time to see what it really sad. Of course I pressed the Upgrade option to see what I would get. Don't worry you can't 'upgrade' a newer version to an older version.
I think we have a programmer who is making use of some variables and he didn't for see this.