Contrassegno del barista
(Mark of the bartender)
A rare sight, a dribble on the side of your demitasse indicating that you've received a perfectly extracted espresso ristretto.
The barista removes the demitasse from the stream before turning off the pump (on purpose mind you). If done at the correct time, the dribble will be thick and full like dark crema, but if done too late, you get a light colored, runny mess that looks like a mistake.