I waited for the other car to go, since it had arrived first. The street onto which I was turning is narrow and cars park on the right side of it (relative to the way I was turning), so I wanted the other car to go first, anyway, since otherwise I would have to swoop around it rather than simply turning directly into the space it currently occupied. But the driver of the other car gestured for me to go, and at this point I saw another car approaching behind it as well and knew that I wouldn't be able to avoid swooping, so I just went.
Now, the other driver probably considered his actions to be polite. After all, he let me go when he didn't have to. However, I realized afterwards that I actually considered his behavior to be (very mildly) rude, because: 1) he was legally in the wrong—since he came to a stop at his stop sign first, he should have gone first; 2) everyone was (anticipably) worse off because of his actions—I had to swoop, he had to wait for me to pass, the car behind him had to wait for both of us.
Extremely minor occurrence, but I find it interesting because it shows my focus on results rather than appearances.