(From Georgios on the mailing list):

I have a problem with section A.4.4.5 in the GIST draft [...] and in particular
related to the way of how a GIST-DATA message is treated by a stateless GIST
responding node. It will be good to clarify what it is meant, in Section
A.4.4.5, by the term unexpected Data message.

Please note that a stateless  GIST responding node MUST be able to receive a 
GIST DATA message without that this will generate an error situation.

[It needs to be pointed out that the error message is only needed by nodes which
require routing state in the first place, and not by other nodes. This also
needs to be fixed in rule 4 for the Node state machine, section 6.1; A.4.4.5
could also include references to the other conditions, from 6.1 and 6.3.]