That's what I thought, too – until this happened to me only two weeks ago. I'd already accepted a status challenge for AS, and they matched at MVPG, but I knew I'd have to forgo my AA upgrades – that was where I changed from – for the time being. I'm not entirely clear what happened, except I got to the airport to discover AA had cancelled its normal 9:50pm service (for DFW-AUS) but left the 10:50pm flight intact. I already had a seat assignment & everything, but upon arriving at the check-in counter I saw a total of 27 people on it – on a Saturday night, no less! (though admittedly this was one of the days when bad weather had a rollover effect on everything).

I can only guess that there were only 3-4 AAdvantage elites on the entire flight, so they started adding Oneworld elites instead. (F was barely half-full by the time they initially boarded the cabin, and I'd already been upgraded a half-hour earlier.

Probably an anomaly, but still.