When submitting a challenge response, Let’s Encrypt responds with a validation status of “valid”, “invalid”, or “pending”.
Let’s Encrypt determines this status by hitting the preagreed endpoint and looking for proof of domain ownership.
If that endpoint returns X, Let’s Encrypt will give validation status Y. (shown as X: Y)
Status code 200, correct body: "valid"
Status code 200, wrong body: “pending”?
Status code 404: “pending”? “invalid”?
Status code 502: “pending”?
Can anybody clarify the 404 case specifically? I’m pretty unsure about most of the cases so if somebody has deeper knowledge or could point me to the specific code paths in the server that would be appreciated.
 I think it’s “pending” but it might be a different word. In any case my experience has been that this state allows Let’s Encrypt to poll for the endpoint until it becomes available (potentially with some short timeout).