That they don't have a healthy sense of humor.

I remember a comedian who toured Europe, and said it wasn't true - Germans laughed as much as anyone else at his jokes. However, afterwards they took him aside and explained, "It was very funny, you see, the joke about combs being like salad forks, but we just want you that we don't discriminate against forks here. That was an unfortunate incident from during the War, but today we are much more enlightened and invite all kinds of cutlery!"