actually this happens quite often when converting poly to subD and using the undo command. might be a bug or somethnig but what you got is two shapeNodes - a subtessnode+polynode at once. i cant tell specifically cause i have no maya installed here to proof. but usually you´d want to delete the subD_node. in this case tho that leads to deleting the complete mesh. so you have to delete the polygonal shapeNode and convert the subD_node back to poly by using the convert command with control vertices checked.

happend to me a hundred times.