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Sometimes, dealers will keep a car on their site in the hopes someone will respond to that listing.
"Oh, It just got sold, sorry. But, we have a sale on Sentras!"
From similar incidents on other cars, I'm guessing a tranny software issue.
Depending on your throttle input and, if you retarded your throttle near or at the point of a gear change, the tranny may have been deciding whether or not to change gears.
Hummer - letting a customer leave with or without a car on an insanely stupid deal you can't do, so when they come back they've already fell in love with the car in their driveway, shown to friends, etc.
There is a procedure to electronically file a report to the manufacturer that a unit has been sold. It's called an "RDR", or Retail Delivery Report.
Part of the processing of a sales transaction is also to remove that unit from inventory (which includes their website), in part to also advise...