When test-pulling 3 pages (of qplimit size 3) on the qppage Wantedfiles page (action: query & list: querypage). It tended to return 8, instead of the expected 9, unique entries. (total wantedfiles entries is greater than 9)
Comparing the returned data per call, the missing 9th case is due to the fact that the first and second call seem to have a tendency to return a duplicate entry. Only significant (so far) difference between the first and second call is the the first call is not yet using the qpoffset setting (second call is using qpoffset is 3)
Properly not really critical, as long as the session pulls all entries (eventually).
Unknown (at this point) if this is a more general querypage thing, or just limited to the Wantedfiles case.
MediaWiki:1.25.2 PHP:5.6.7-1 (fpm-fcgi) MySQL:5.5.43-0+deb8u1
(more NTS then anything else)
Update: Definitely less benign than on might think. Getting only 75 unique entries back after 5 calls, of size 25 on a 107 wantedfiles list, kinda triggers a returned-data credibility issue ... well, to some.