The border is defined as a straight line, but was surveyed as a series of slightly wobbly series of lines. They put up thousands of stone pillars, cut out a strip of trees along the border, and it became official.
IIRC there's also some stuff about a disputed island
I once visited Port Roberts just to see the border crossing. I drove in from the Canadian side, parked my car, and walked across the border into the U.S. without being challenged. No one asked for my passport. As far as I could tell, no one even noticed that I had crossed the border. But when I tried to walk back into Canada the Canadians gave me a pretty hard time and almost didn't let me back in. It was an interesting experience.
