I had a shotgun aimed at me, once, while photographing a tree which was on a private farm. I was, however, not on the property.
I was detained in the basement of the US Capitol, once, while waiting for the person who issued permits to get back from lunch, so she could tell the police I had a permit.
One time, there was almost an international incident, while doing some work for a DC real estate management company. I was hired to photograph their portfolio, which included embassies, office buildings, and apartment buildings. For most of the buildings, I had to shoot interiors and exteriors. For a particular Middle Eastern embassy, I could only shoot from the street. Apparently, the security staff didn't get the memo I was coming (it was prearranged to avoid any issues) and saw me shooting the building. About 8 guys, with guns, came running through the gate, and forced me to my knees. While on the ground, a DC patrol car came upon the scene, called for backup, and before long there were about 5 other patrol cars there. After some discussion about jurisdiction (the security guys wanted to detain me inside the gates, and confiscate my film/camera), a higher up came out to say he had forgotten to mention that I was expected. The security withdrew, and one of the cops said, "If you had gone in, there isn't much we could have done, but they probably would have let you go, eventually..." Scary then, neat story now.