Extension dragons

draft-ietf-quic-transport.md

@@ -5710,6 +5710,12 @@ that a peer is able to understand the frame.  An endpoint can use a transport
 parameter to signal its willingness to receive one or more extension frame types
 with the one transport parameter.
 
+Extensions that modify core frame types or define alternatives to replace core
+frame types will be difficult to combine with other such extensions which modify
+or replace the same frame(s) unless the behavior of the combination is
+explicitly defined.  Such extensions SHOULD define their interaction with
+previously-defined extensions modifying the same protocol components.
+
 Extension frames MUST be congestion controlled and MUST cause an ACK frame to
 be sent.  The exception is extension frames that replace or supplement the ACK
 frame.  Extension frames are not included in flow control unless specified