Originally Posted by
ISTFlyer
I am %100 sure that United won't sell $139 walk-in tickets for a transcontinental flight on a Sunday which was fully booked at the time we boarded.
Keep in mind it is primarily the advance purchase requirements written into the fare rules and not individual fare bucket availability that prevents this from occurring. It is not uncommon to see all fare classes open close to departure even up to G9 which would allow a free SDC on any fare while the cheapest fare available for sale may be close to full fare.