Stupid USA, meeting Germany in Düsseldorf, but not asking für Düsseldorf beer...Some facts:
Germany has around 186 different brands of beer (beer mixed with other alcohol or juice not counted in) and - because of the German purity law, which defines how a beer has to be made - most of those are said to be the best beers of the world.
That would be all fine, but Germany is also very proud and thus special about it's beer.
Many regions have their own kinds of beer and will take it personal, if you ask for a beer that can be bought anywhere. Or - even worse - when you ask for a beer from a rival.More facts:
The most famous rivaliry is between Köln (Cologne) and Düsseldorf. Asking for a Düsseldorf brand in Köln or the other way might bring a lot of trouble or even lead to you being thrown out of the shop/bar with lifelong prohibition to re-enter. Even though foreigners are usually treated a bit kinder, where the shop owner just plays dumb until he can "recommend" the brand that the town is famous for. But you should be so wise to say how tasty that brand is that you got: they might otherwise think that you don't like their beer.
(Comic strip based on Monty Pythons "Cheese Shop Sketch"