As an occasional slot player, I often walk away from a machine without taking out my card. Its only when I sit at the next machine, I realize it, and have to go back looking for the machine. It hasn't happened but I could easily find a card in a machine like the one I was playing and take it without checking it.
I can't imagine a casino turning you down for a jackpot because another persons card was in the machine. The case of the two women was different as they tried to say it was the one person when the video showed it was the other woman.