Made multires and shape keys throw errors when adding if the other existed (so both can never coexist) documented this too.

source/blender/python/api2_2x/Mesh.c

@@ -6323,6 +6323,10 @@ static PyObject *Mesh_insertKey( BPy_Mesh * self, PyObject * args )
 	char *type = NULL;
 	short typenum;
 	
+	if (mesh->mr)
+		return EXPP_ReturnPyObjError( PyExc_RuntimeError,
+					      "Shape Keys cannot be added to meshes with multires" );
+	
 	if( !PyArg_ParseTuple( args, "|is", &fra, &type ) )
 		return EXPP_ReturnPyObjError( PyExc_TypeError,
 					      "expected nothing or an int and optionally a string as arguments" );
@@ -7301,6 +7305,9 @@ static int Mesh_setFlag( BPy_Mesh * self, PyObject *value, void *type )
 			}
 		} else {
 			if ( !mesh->mr ) {
+				if (mesh->key)
+					return EXPP_ReturnIntError( PyExc_RuntimeError,
+						"Cannot enable multires for a mesh with shape keys" ); 
 				multires_make(self->object, mesh);
 			}
 		}

source/blender/python/api2_2x/doc/Mesh.py

@@ -787,6 +787,7 @@ class Mesh:
   @type activeColorLayer: string
 
   @ivar multires: The mesh has multires data, set True to add multires data.
+    Will throw an exception if the mesh has shape keys; use L{key} to test.
   @type multires: bool
   @ivar multiresLevelCount: The mesh has multires data. (read only)
   @type multiresLevelCount: int
@@ -1057,6 +1058,7 @@ class Mesh:
         seems that 'frame' should be kept in the range [1, 100]
         (the curves can be manually tweaked in the Ipo Curve Editor window in
         Blender itself later).
+    @warn: Will throw an error if the mesh has multires. use L{multires} to check.
     """
 
   def addUVLayer(name):