Original text:
9.5.20. <del> The 422 (Unprocessable Entity) status code indicates that the server understands the content type of the request entity (hence a </del>  <del> status code is inappropriate), and the syntax of the request entity is correct but was unable to process the contained instructions.  For example, this error condition may occur if an XML request body </del>  contains well-formed (i.e., syntactically correct), but semantically <del> erroneous, XML instructions. </del>  9.5.21.

Reviewer comments:
See ; the issue is not just about moving the text.
I understand that, I just thought it would be good if we could split this into two changes; that makes it easier later on...
See - I'm on it.
I think I'm ok with the actual changes, but we need a summary for the "Changes from RFC 723" section. Proposal:
NAME I think this is ready to go.
We still need to document this as a normative change from RFC 723.
Done.
LGTM.
As discussed quite a bit, retries in HTTP need to be better defined. In particular, HTTP does not offer a guarantee that any particular request will not be automatically retried.
Messaging - 9.4.1. Retrying Requests contains some things that probably belong in Semantics, probably somewhere up in the architecture section. Beyond that, I think the action here is to clarify a bit more that POST etc. requests are actively retried by many UAs (e.g., most Web browsers) because of H1's connection management issues. NAME can you confirm that that's H1-specific?
generally goaway would determine this in h2
Semantics - 7.2.2 Idempotent Methods talks about retries some. I'm inclined to add a very short reference to it in architecture -- e.g., "Some requests can be retried in the event of failure; see ref" -- around the sentence starting "A client sends an HTTP request..." in 2.1 Client Server Messaging. In h1-messaging, I'd like to add the behaviour that UAs actually implement to the list of examples of how they'd know it's OK to retry. AIUI, that would be something like: NAME is that accurate?
Well, the server could have had an opportunity to process a request, because they routinely start processing based on only a small amount of the header. The fact is that some clients retry based on very weak signals, risking duplicate processing in favour of greater robustness. The retry logic as I understand it is "have I seen any hint of a response? no? retry."
It seems like either clients or the spec should change, then. I suspect clients will not...
There's two things you might consider here: noting that clients might do some things to retry based on their own determination of when that is appropriate (for example, in the extreme, retrying if the connection breaks before any byte of a response is seen).recommendations about good practice, which probably doesn't want to point at what the most aggressive clients (i.e., browsers) might choose to do Trying to walk the line without making that distinction is probably too hard.
That seems like a good direction, except that the spec currently contains a MUST NOT; "good practice" isn't that strong... MUST NOT (BUT WE KNOW YOU WILL)?
Yep. Part of this work is in aligning the spec with reality. A SHOULD NOT is probably appropriate. No need to resort to RFC 6919 language.
Proposal:
LGTM. s/; f/. F/
While I'm fine with this, I'm stil having a big problem with retries. Some of our (haproxy) users are pressuring us to implement the so-called "L7 retries" by which if the intermediary faces a problem with the server after the request was delivered, it happily retries it, even for a POST! Not only it's a technical problem (requires all requests to be buffered for the time it takes a server to respond) but it's in total violation of all rules. The problem here is that there's an ongoing trend for doing this and users find it normal since it allows them to hide their constantly crashing servers... And usually they consider that the application deals with replay protection by itself so obviously the infrastructure must retry. Thus my big concern here is to figure till what extent an intermediary could reasonably retry, or how we could have signals between UA and server to indicate if the request is retry-safe.
A protocol extension seems reasonable for that case; I think it could be interesting to clients -- both intermediary and otherwise.
Just leaving a note to remember to go through to make sure we got everything.
I had a bunch of editorial suggestions to make and decided to just make them directly to the pull request to save you the time and annoyance. I hope that is okay, since my changes just reduce the prior diffs.

Revised text:
9.5.20.  <ins> The 422 (Unprocessable Payload) status code indicates that the server understands the content type of the request payload (hence a </ins>  <ins> status code is inappropriate), and the syntax of the request payload is correct, but was unable to process the contained instructions. For example, this status code can be sent if an XML request payload </ins> contains well-formed (i.e., syntactically correct), but semantically  <ins> erroneous XML instructions. </ins> 9.5.21.