These can really be a (edited) to deal with. There are two ways that immediately come to mind. These can be otherwise known as 'empty groups'. You can purge them (eg. using CDGpurge 3.0) or rename the missing block reference. I suspect them of causing various problems in AutoCAD, such as crashes when attempting to bind xrefs.
Other users forthcoming responses will more than likely be more informative than myself.