i think it's becoming common. someone placed the wrong price tag, customer found it and tried to get it for the price. if they don't get it, they cause a huge fuss and in the end, the store just wear the loss than to deal with adult tantrums. the manager wouldn't have known how you would react if he tells you that you had to pay full price, so just just honor the price on the shelf.
once i bought a bag of rice at the supermarket, which had 50% discount label on the shelf. when it was scanned, i was charged full price. went to the service counter, they confirmed someone forgot to remove the 50% discount tag from the previous week. they removed the tag, refund the $$ i paid. and i get to keep the bag of rice for free. *shrug* granted, it wasnt a mavic pro.