[This one is true - or maybe apocryphal? I have embroidered the details - if anyone knows the unadorned truth I'd be happy to be corrected.]
Sibelius, already famous, was staying in Hotel Fennia in Helsinki in early 1911, trying to work on the Fourth Symphony. He was disturbed by a beggar in the street playing the accordion very badly; plainly he had earned no money with his playing.
After a while Sibb could take it no longer and went down to see the beggar. Moved by the poor man's insistence that playing the accordion was his only means of earning a crust, Sibelius took pity and showed him a few easy ways to make the sound slightly less awful... and then set off for a quick snifter at the Kämp.
On returning to the hotel some XXX hours later, Sibelius was greeted happily by the same beggar again. But this time his hat overflowing with coins - and there was a handwritten notice hanging around his neck: 'PUPIL OF JEAN SIBELIUS'.