The One With The Inappropriate Sister - S5-E10
Corrected entry: Phoebe is collecting for charity and Monica comes to see her; a man puts a dollar in the bucket and tries to make change for the bus. When Phoebe argues with the man he says "bite me blondie", but he couldn't have known that she is blonde because she has a Santa hat on that covers her entire head and no hair is coming out from underneath the hat.
Correction: I thought this too, but it's clear in the next shot, when she turns slightly, that Phoebe's long hair comes out below the hat and hangs right down her back - this would have been evident from the man's point of view, even if it's not evident from the front, and therefore to the viewer.
Correction: Her hair is simply not out when the guy is there. It is simply a mistake. Her hair is out in the next scene she appears in but not one little bit is out when the guy says "bite me blondie." Only way he could know is if he has seen her before and recognised her.