This almost happened to me last weekend, in my own yard. Here's what happened. I live in a small apartment building - four units, so there's little parking. On Sunday, my neighbor's father and brother paid a visit and parked in the only space available - the front yard.
Just as they got out of their cars, two other cars crashed at the intersection, and the foot of one of the drivers slipped off the brake and hit the gas. His car crashed into my neighbor's father's truck and slammed it into her brother's car. The front wheels of the car continued to spin and dug deep holes in my yard.
The driver of one of the vehicles began to blame my neighbor - saying the cars should not have been parked where they were, so I went inside, got my Nikon F and a roll of film, and began taking pictures to document what happened.
The police went into hysterics. One ran up to be and screamed in my face. I had to ask him to repeat himself three times, telling him I couldn't understand him when he yelled like that. I finally understood that he was saying I didn't have any right to be there. I pointed out that it was my yard, I had every right to be there.
Then he threatened to arrest me for "interfering with my accident scene." I told him I'd sue his ass off. He ordered me to leave, so I went inside and took photographs of the police standing around doing nothing through my window.
We seem to have a rash of stories about police objecting to photography with real cameras while ignoring the true culprits - digital and cell phone cameras.