My experience last week was this:
Trying to book an M fare ATL-LHR-ATL (stopover) - DCA. The fare allowed the stopover (for $50 extra), and the website would price it correctly, but when attempting to go to the next page I would get the infamous "sorry that fare is no longer available" error screen. Looking at ExpertFlyer, I knew there was plenty of M inventory on all legs, so the fare was, indeed, still available.
After totally failing to comprehend the problem, the live chat agent told me that if he/she booked it for me I would be charged the $20 direct ticketing fee.
I declined and called SMS instead, where of course the itinerary priced correctly and where, of course, I had to pay the $20 to have something done that the website could not do.
I think the stopover was what threw the system into dysfunction.