Document the limits to MoveTransition, check for errors.

renpy/display/movetransition.py

@@ -455,6 +455,11 @@ def MoveTransition(delay, old_widget=None, new_widget=None, enter=None, leave=No
     Returns a transition that interpolates the position of images (with the
     same tag) in the old and new scenes.
 
+    As only layers have tags, MoveTransitions can only be applied to a single
+    layer or all layers at once, using the with statement. It will not work
+    in other contexts, like ATL, :doc:`ComposeTransition`, or other ways of
+    applying transitions.
+
     `delay`
         The time it takes for the interpolation to finish.
 
@@ -488,6 +493,14 @@ def MoveTransition(delay, old_widget=None, new_widget=None, enter=None, leave=No
         A time warp function that's applied to images leaving the scene.
     """
 
+    if not (hasattr(old_widget, 'scene_list') or hasattr(old_widget, 'layers')):
+        if renpy.config.developer:
+            raise Exception("MoveTransition can only be applied to one or all layers, not %s." % type(old_widget).__name__)
+
+    if not (hasattr(new_widget, 'scene_list') or hasattr(new_widget, 'layers')):
+        if renpy.config.developer:
+            raise Exception("MoveTransition can only be applied to one or all layers, not %s." % type(new_widget).__name__)
+
     use_old = old
 
     def merge_slide(old, new, merge_slide):

sphinx/source/transitions.rst

@@ -43,6 +43,9 @@ statement. For example::
     their new locations. An instance of the :func:`MoveTransition`
     transition class.
 
+    Move transitions, and similar transitions like ease, can only be applied
+    to one or all layers, not individual displayables.
+
 .. var:: moveinright
 
     Also: **moveinleft, moveintop, moveinbottom**