In certain cases (yet to be determined), stashing transform requests has a mismatch in the revid between the ETag and URI parameter. In all of these cases, the request is sent to /transform/html/to/wikitext/{title}/{revision}, but the ETag is 0/{timeuuid}/stash. This suggests that in a previous call RESTBase set 0 for the revision id when it should have set it to the appropriate value, since these requests are not for new pages.
An example request is for /es.wikipedia.org/v1/transform/html/to/wikitext/Mar_de_amor/120200307 sent with the If-Match: W/"0/d14208f0-eec3-11e9-9a9b-fd292f60a221/stash" header. RESTBase responds with a 404 because it can't find the stashed content with revid 120200307:
cassandra@cqlsh:wikipedia_T_parsoidd3o5Dn1wcj_Xve2tXe4_rtmeWSU> select key,headers,tid from data where "_domain"='es.wikipedia.org' and key='Mar_de_amor:120200307:d14208f0-eec3-11e9-9a9b-fd292f60a221'; key | headers | tid -----+---------+----- (0 rows)
Alas, the content is there, just with revid 0:
cassandra@cqlsh:wikipedia_T_parsoidd3o5Dn1wcj_Xve2tXe4_rtmeWSU> select key,headers,tid from data where "_domain"='es.wikipedia.org' and key='Mar_de_amor:0:d14208f0-eec3-11e9-9a9b-fd292f60a221';
key | headers | tid
----------------------------------------------------+-----------------------------------------------------------------------------------------------+--------------------------------------
Mar_de_amor:0:d14208f0-eec3-11e9-9a9b-fd292f60a221 | {"etag":"\"0/d14208f0-eec3-11e9-9a9b-fd292f60a221/stash\"","content-type":"application/json"} | d283fed0-eec3-11e9-9a9b-fd292f60a221
(1 rows)