Some days ago I had an unpleasant experience with a security guy. I was taking photos of a skyscraper in a public area in Hamburg. After I aimed my camera at the building I heard him shouting after me from three meters away. After I turned around he came closer telling me that I need a permission from the owner to take a photo of this building.
I asked him if he knows the German media laws and that there is no need for a permission for various reasons. He just repeated the same phrase over and over again. I had a bit of a short temper this day and told him that he behaving not in a polite way and that he is pestering me. I left the scene without listening to his angry voice anymore.
This incident showed me how important it is to know your rights as a private or commercial photographer. Maybe you get into a similar situation with someone more important than the security staff or the police. When you know your rights you can argue better and stay cool.