I have seen this around 6 months ago but can't remember the exact cause. Do you have more information on this maybe it'll jog my memory as to what caused it? For exmaple:
What version of AutoCAD are you running?
Is it consistently repeatable when you run a certain commands?
Have you tried logging a Support Request with Mentor as they do keep a record of customer reported problems?
I have not seen the error since I posted this nor do I remember the details of what I was doing when it happened. Hopefully it won't happen again... If it does I'll be more diligent in submitting a support request.
It happens for me at least once a day. Typically, it happens when I'm placing a connector or splice, between placing the Node and adding the connector, or splice graphic block.
AutoCAD 2008 seems to be able to save the drawing at the exact moment of the error so I'm able to restore everything up to the moment the error occurs.