We just came off the Volendam a few weeks ago and this came up during the cruise. Thought I'd pass this little tip along.....
I bought a wine card and on the second day we had only had used it for a total of 2 glasses of wine. In the diningroom that night I ordered a glass of wine with dinner. Much later that night when I went to order another glass in a lounge, I discovered that my wine card had 8 glasses punched/recorded on it. The diningroom wine steward had given me someone else's card back.
I talked to him the next night and he accepted right away that he had made a mistake and agreed to make up the difference for me. Actually he MORE than made up the difference, but that's beside the point:-) Had he not been so straightforward, it would have been a bit of a disappointment.
So, my tip for wine/soda card holders is to make a mark or other means of identifying your own card so you know you're getting the right card back.