While in Cannes, France, I thought it would be wise to stock up on stamps. We were mailing a bunch of postcards to our family and friends, and I knew that we would be sending a dozen or two more. So I asked for 20 stamps to start us off. We promptly used 10, and I figured that the next 10 would hold us another week. Except I forgot one detail: we were leaving France. With the bulk of Europe using the Euro and the seamless inter-country transit, it completely slipped my mind that the countries don’t use the same postal system!
So now I am the proud owner of 10 French international stamps that we will likely never use, at a cost ~$14. I hate it when I make unnecessary purchases that negate all the ways in which we save money while traveling, but sometimes they happen. You live and learn.