Here's an interesting one where the details will have been lost on most modern viewers.
but first let's get this out of the way: Gulliver Dark, seen singing at the Sapphire Lounge, also performs with Tito Puente's band:

and can be seen in a picture on the wall at Fat Tony's club, implying a mob connection.

But what I found interesting is that he is not voiced by any of our regulars, and is voiced by Sam McMurray (whose only other appearances on the show were a couple of minor characters in Homer's Odyssey.) This is because Gulliver Dark is a character that McMurray played on the Tracey Ullman Show.

He's been redesigned for a cartoon, but it appears that it was meant to be the same character, a minor crossover between the Simpsons and the Tracey Ullman Show. In 1990 this was the equivalent of Nick Fury showing up and the end of a Marvel movie, I'm sure, but is lost to time now - I can't even find how regular a character Gulliver was on the Tracey Ullman show, his Simpsons appearance has eclipsed it.
Obviously Dan Castellaneta and Julie Kavner were cast members on the Tracey Ullman show, as well as the Simpsons themselves, perhaps McMurray was considered for minor recurring characters before Hank Azaria and Harry Shearer essentially filled those roles. At least he'll always have Gulliver Dark.*
*except for his other speaking appearance, when his singing voice is performed by George Balmaseda.
This post is part of my "Every Simpsons Character Ever" series. For a list of my rules in this project, click here.