The way it started was that the third player in our flight was located on the fringe of the green. Because he was unable to address his ball properly with my ball in the way of his left foot, he asked me politely to mark my ball.
So that is exactly what I did. Without thinking much of it, I proceeded unconsciously out of habit to clean my ball with a couple slight rubbing movements of my palm and fingers.
I figured that since my ball was overhanging the green half way that the usual rules of golf would apply, therefore allowing me to clean my ball after marking it.
However, although to the casual observer it may have appeared not unreasonable to assume that the ball was actually on the green, the official rules of golf are very clear and strict about this oversight of mine.
According to the official rules, the ball is only deemed 'on the green' when any part of the ball is 'touching' the green.
Overhanging half way is NOT the same as touching the green. My opponent was correct, quite observant, and he could have easily incurred the penalty on me. Even though I was requested to mark and pickup my ball, I was not allowed to clean it in any way whatsoever, not even a subtle rubbing motion of my fingers.
Although this action did not lead to any bad feelings or serious altercations towards each other, I was shaken up enough about the whole affair that I proceeded to double bogey the next two holes.
Thereby giving myself the penalty strokes I probably deserved anyway, even golf is not immune to the law of retribution.
That is why the game of golf is an interesting sport in that regard. I've learned my lesson and won't forget it the next time.