Every so often when others in the office and now, today, myself have been editing an x-ref in place it will not allow us to save the changes we have made. 
  
First when you try the dialogue box flashes up , as attachment.  I press F2 to bring up the command line box and have copied the relevent lines below. 
  
Command: _refclose 
The following symbols will be permanently bound to the current drawing: 
Text Styles: $0$Arial 
Blocks: $0$_ArchTick 
  
  
Enter option [save/Discard reference changes] : _sav 
Regenerating model. 
Errors found in references to other objects: 
** Object reference missing: AcDbBlockReference, to AcDbDimAssoc. 
Regenerating model. 
  
*Cancel* 
  
Any ideas how to work around this or correct it?  Luckily on this occasion I was simply stretching a kitchen to fit a new layout but others have lost an hours work (I have no idea why they were in there for so long either:?)