I was on my way home on the tube last week, and there was a man sitting opposite with a sketch-book, drawing some of the passengers on the train.
There was a man sitting on one side of me, who was big, with a bald head and huge black sunnies, completely oblivious to the fact that he was being sketched.
When he'd finished drawing him, he started drawing the man sitting on my other side (I was only slightly offended not to be a worthy subject!). This guy noticed, and looked a bit amused. When he'd finished, he gave the drawing to his subject, who looked quite chuffed to receive it.
Just another great London moment.