join could crash blender in background mode or if the mesh was not in the current scene. added exceptions for both and notes in the EpyDocs.

source/blender/python/api2_2x/Object.c

@@ -2025,6 +2025,10 @@ static PyObject *Object_join( BPy_Object * self, PyObject * args )
 	short type;
 	int i, ok=0, ret_value=0, list_length=0;
 	
+	if( G.background )
+		return ( EXPP_ReturnPyObjError( PyExc_RuntimeError,
+						"cannot join objects in background mode" ) );
+	
 	/* Check if the arguments passed to makeParent are valid. */
 	if( !PyArg_ParseTuple( args, "O", &list ) )
 		return ( EXPP_ReturnPyObjError( PyExc_TypeError,
@@ -2049,12 +2053,16 @@ static PyObject *Object_join( BPy_Object * self, PyObject * args )
 		return ( EXPP_ReturnPyObjError( PyExc_TypeError,
 						"Base object is not a type blender can join" ) );
 	
+	if (object_in_scene( parent, G.scene )==NULL)
+		return ( EXPP_ReturnPyObjError( PyExc_TypeError,
+						"object must be in the current scene" ) );
+	
 	/* exit editmode so join can be done */
 	if( G.obedit )
 		exit_editmode( 1 );
 	
 	temp_scene = add_scene( "Scene" ); /* make the new scene */
-	temp_scene->lay= 2097151; /* all layers on */
+	temp_scene->lay= 1; /* all layers on */
 	
 	/* Check if the PyObject passed in list is a Blender object. */
 	for( i = 0; i < list_length; i++ ) {
@@ -2069,6 +2077,13 @@ static PyObject *Object_join( BPy_Object * self, PyObject * args )
 			/* List item is an object, is it the same type? */
 			child = ( Object * ) Object_FromPyObject( py_child );
 			if (parent->type == child->type) {
+				
+				if (object_in_scene( child, G.scene )==NULL) {
+					free_libblock( &G.main->scene, temp_scene );
+					return ( EXPP_ReturnPyObjError( PyExc_TypeError,
+						"object must be in the current scene" ) );
+				}
+				
 				ok =1;
 				/* Add a new base, then link the base to the temp_scene */
 				temp_base = MEM_callocN( sizeof( Base ), "pynewbase" );

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

@@ -675,10 +675,13 @@ class Object:
     @note: Join will only work for object types Mesh, Armature, Curve and Surface,
         an error will be raised if the object is not of this type.
     @note: objects in the list will be ignored if they to not match the base object.
+    @note: objects must be in the current scene to be joined.
+    @note: this function will not work in background mode (no user interface)
     @note: An error in the join function input will raise a TypeError,
         otherwise an error in the data input will raise a RuntimeError,
         for situations where you don't have tight control on the data that is being joined,
         you should handel the RuntimeError error, litting the user know the data cant be joined.
+        This an happen if the data is too large or one of the objects data has a shape key.
     """
 
   def makeParentDeform(objects, noninverse = 0, fast = 0):