docs for KEYDOWN post behaviour

docs/reST/ref/event.rst

@@ -346,6 +346,13 @@ Events reserved for :mod:`pygame.midi` use.
 
    If the event queue is full a :exc:`pygame.error` is raised.
 
+   Caution: In pygame 2.0, calling this function with event types defined by
+   pygame (such as ``pygame.KEYDOWN``) may put events into the SDL2 event queue.
+   In this case, an error may be raised if expected properties of that event
+   (defined in the table at the top) are missing, and unexpected properties may
+   be silently omitted. In order to avoid this behaviour, custom event
+   properties should be used with custom event types.
+
    .. ## pygame.event.post ##
 
 .. function:: custom_type