I have been getting this message randomly in Revit 2011 MEP for tags when moving objects around. For example, I have a lighting fixture with a circuit tag and a fixture type tag. When I use the move command and pick start and finish points, the fixture moves, then I get the warning and it deletes the two tags. Looking into it further, it looks like it is created a new object as the mark number changes (which would make sense why the tags would be deleted). Has anyone else had this problem with phantom element deletion/creation when moving around object? Anyone know of a solution? I have to update 30 floors of minor changes and it is deleting all the tags.