This can happen to a flight that is delayed by technical problems.
Passengers on that aircraft might be allocated seats on other flights, but the delayed aircraft still has to get to its next assigned port to fit into airline schedules.
Mass cancellations in the aftermath of a disaster can also cause aircraft to fly close to seats.
In the immediate aftermath of 9/11, passengers reported near-empty flights departing from US airports.
In another case, during blizzards in China in February 2016 just one passenger flew aboard a flight from Guangzhou to Wuhan.
While it might seem intuitively obvious for airlines to have a fire sale of empty seats close to flight time that's not the way they operate.
As the cheaper earlybird seats are sold, flyers are left with more expensive options.
Airlines earn a disproportionate amount of their revenue from these late-booking passengers.
If an airline was to discount seats at the last minute to fill them it would jeopardise that lucrative revenue stream.