...says My SpyWitness: "Kendall had her car washed and gassed at Auto Spa in Calabasas, but couldn't pay because she'd left her wallet in another bag - so she asked the woman behind the counter to let her take her car home, and she'd come back and pay.

When the woman refused to hand over her keys, our barely-legal star hissed, 'I'm Kendall Jenner...one of the Kardashian sisters!'" Genuinely confused, the woman responded: "Hello, nice to meet you. So...you can have your car when you pay."

Obviously upset, Kendall phoned a friend, then plopped down and pouted for half an hour until the pal  pulled up with her wallet. Yanking out a credit card, Ms. Superstar huffed and puffed while the woman ran it, then scribbled her name and stormed out.