Originally Posted by
Zembla
I don't think it was an error of the person who asked you to pay for a product, but it was an error of the cabin crew. However, you have paid for something which they were giving away for free only moments later. Call them and ask your money back.
Spot on.

Cabin crew should not allow Economy passengers to move into Economy Comfort. I would simply demand a refund from KLM, and I think chances are pretty good you will get one (and the cabin crew of this particular flight will get some additional training).