One company was making two different kinds of wine, which had very similar taste. One sunny day 3 friends were sitting outside and drinking one of the wines. Kostya walked up to them and bet that he could tell which wine is which. They made a deal. The friends tied his eyes, brought a bottle of the other wine, and let him drink a cup of each wine. Kostya guessed which wine was which. And the three friends would have paid Kostya if an old man had not passed by. He said that the bet wasn't fair.

What was unfair about the bet?