Warn clients about changes in event format

specification/client_server_api.rst

@@ -1267,7 +1267,21 @@ point in time::
 
   [E0]->[E1]->[E2]->[E3]->[E4]->[E5]
 
+.. WARNING::
+
+   The format of events can change depending on room version. Check the
+   `room version specification`_ for specific details on what to expect for
+   event formats. Examples contained within the client-server specification
+   are expected to be compatible with all specified room versions, however
+   some differences may still apply.
+
+   For this version of the specification, clients only need to worry about
+   the event ID format being different depending on room version. Clients
+   should not be parsing the event ID, and instead be treating it as an
+   opaque string. No changes should be required to support the currently
+   available room versions.
 
+.. _`room version specification`: ../index.html#room-versions
 
 Types of room events
 ~~~~~~~~~~~~~~~~~~~~