Completely avoiding conflict between differing versions of a product installed on the same workstation is tough...just ask Autodesk!
Those who install AutoCAD 2013 but need to return to a prior version of AutoCAD from time to time may encounter a mysterious dialog when they exit such prior version of AutoCAD. The dreaded empty dialog aka exclamation only dialog! No words, just a scary exclamation mark and an OK button. Other particular installation/removal activities can also cause the same issue.
Fear not, your save on exit went just fine & there is no danger to your drawing or your AutoCAD configuration. Just click OK and move on with your life. The lucky ones will see this twice each time.
The condition under which this dialog appears seems to be some unforeseen interaction between something installed by AutoCAD 2013 that interacts unfavourably with a prior version of AutoCAD wherein some extra executables have been added, like but not limited to DraftLogic Electrical.
See this Autodesk forums entry for the variety of situations people have encountered if you are interested: http://forums.autodesk.com/t5/AutoCAD-Civil-3D/C3D2012-is-giving-me-the-Silent-Treatment/td-p/2995970
Net take-away from this forums entry is to not fear or be concerned about the 'exclamation only dialog'...don't worry, be happy.