You buy an item as cash on delivery. Flipkart will prod you to pay online before delivery. You make the online payment before delivery.
The delivery man will come and ask you for cash. When you say that you have already paid online, he’ll say that he doesn’t have that information and he needs the cash to deliver the product; and, will go away without handing you your ordered item for which you have already made the payment.
You then talk to Flipkart. They will tell you that you have to pay the delivery man for the item and then get a refund of the online payment later!