Just to throw an anecdote onto the pile, I (carelessly) totally ignored the detail that the TATL I'd booked at aa.com (out of FCO) remained 'on request' until I tried to OLCI.
The message from BA.com didn't sound dire - "call our agent, or simply show up at the airport where we'll sort it out" - so when we arrived at FCO I walked up to the BA office to ask what was the problem. "We have your booking and are holding space, but don't have ticket numbers [and neither our wonderful ManyWorld Alliance nor our joint operating agreement with AA allows us to retrieve them, so go talk to AA about it.]
I quickly logged into aa.com to retrieve the ticket numbers only to learn that the booking was 'on request' and had been since June 4, 2013 [sic]. My fault for not checking on June 6th (2012), their fault for a computer glitch that got the year wrong and thus probably screwed up the progress of ticketing.
At least with plat I fairly quickly got to an aagent who understood the issue and went to work getting me ticketed, likely some effort on her part. Half an hour later we were good to go. BA's computer still hadn't heard from AA's computer, but the aagent had read me the numbers and the BA rep was pleased to type them in. Amusingly she couldn't simply issue bp's, but OLCI worked.
Another lesson learned the hard way. I had AA's PNR and also BA's, which lulled me into thinking I was good to go. Not exactly.