Well, gender, for one thing. In Germany, Jan (or yahn, as it is pronounced in German) is apparently exclusively for men. I have seen some confused looks on people, but my name hadn't yet caused any problems...until today.
When I give the the chatty lady behind the grocery store counter my debit card, she says, "Is this your card?" I tell her in my broken German that I am American, and that in the U.S. Jan is usually a name for a woman. Her answer is something like, "Wouldn't that be Jane?" Now, it's often difficult for me to discern the tone in which Germans are speaking, so I couldn't tell if she was making conversation (seems hardly likely, considering the number of people in line behind me) or if she was actually arguing with me about my name. I just smile, as I feel my face turn purple, and quickly sign for 6 Euros worth of groceries. She then compares the signature with the one on my card.
Next time, I'll just use cash!