Okay this can happen occasionally with SDs but dont panic its not major.
Usually its best to re convert thru the menu instead of ctrl_z all the time (lazy)
Put a shortcut on a shelf for conversion to and from SD.
Okay back to the problem, go to the hypergraph and select the mesh and show the upstream and down stream connections. You will have an extra poly node in there that needs to be deleted. Also you may have SD blind data nodes in there as well which is basically history of the sd, just delete those, that should rectify the prob