Travelling to different countries means, among other things, adjusting to tiny little regional quirks.
Do you pass on the right or the left?
Do you serve food with your right or left hand?
Which hand holds the fork?
Which hand gives the cash?
When do you say the equivalent of thank you, and when do you say the equivalent of cheers?
How much do you tip a waitress? A cab driver?
Most of these customs I can easily adjust to by being very aware of my actions for a few days, after which I fall into a habit that I then carry back home with me, at which point everyone looks at me funny. Well, not the fork. It took me some time to learn to train my left hand to cut and while my right speared.
There's one I'm having trouble with. In college, a black man told me that when he went shopping, the cashier more often than not put change on the counter for him to pick up. Most of his white colleagues always got cash handed to them. I checked with my friends. My white friends had it handed to them more often than I did. My black friends had it put on the counter more often than I did. Anecdata, I know, but it disturbed me. Since then, I have always made a point of handing cash to everyone. If the bills go on the counter while I am counting change, the coins get put in the hand. If for some reason this is not possible, it is always accompanied with an apology.
In a large chunk of Europe, money exchanging hands directly is just not done. A colleague explained to me that this felt to close to begging somehow. Okay, fine. Different country, different norm. But I can't do it. At international train stations and airports, there's an awkward beat while I hold out the money and the cashier figures out that he/she should take it from my hand and not the counter. At grocery stores and on buses, there's the urgent glances at the coin dish or the embarrassed pushing of change back at me across the counter as I recall that I screwed it up, yet again.