The world's smallest violin
To be sure, there are any number of actual, real very tiny violins, playable or not, that may vie for this title. But this entry is not for any existing violin, but only for a metaphorical one. The "world's smallest violin" is a phrase used in a sarcastic manner to dismiss someone else's troubles as imagined or very minor. Typical use would be accompanied by a hand gesture of rubbing the tips of the thumb and forefinger together in tiny circles and asking "Do you know what this is?" And then replying that's it's "The world's smallest violin and it's playing just for you!"
The metaphor assumes deep cultural familiarity with non-diegetic music - film/TV scoring. But enhancing sad scenes with violin music surely predates the existence of television and film.