Long story short - leaving the shopping center at Randolph Road and Rockville Pike (which contains a Giant Food, among assorted other stores), I turned onto Randolph and stopped at the Pike, waiting for a light change so I could cross over to Montrose Road and head toward 270.
Some time in the last 8 years (i.e., after I stopped being a resident of Rockville), that intersection had at least one significant change made to it. It is now legal to turn left onto the Pike from Montrose Road. This was not true when I lived there. When I saw green light, I reflexively started forward and crossed the Pike, because Mr. Memory said "There can't be any opposing traffic". I was most concerned with checking left and right for cross-traffic, which doesn't always stop when it should; one second or so of time spent actually checking the light pattern would have kept me from making this error.
Fortunately for me and other motorists, there was in fact no opposing traffic, and no accident. Unfortunately for me, a Rockville police officer was behind me, and although he understood how I had made the mistake he was required to cite me for running the red light.
Moral of story: You can easily make bad mistakes while driving in places that you think you "know". Look anyway. Live longer. Be happier.