Frank Zappa appeared as a musical guest on Saturday Night Live in December 1976, performing several tunes. The most interesting was "Purple Lagoon," because it featured an improvised cameo by John Belushi. Belushi hopped into the band, started a call-and-response routine, threatened a member of the horn section with his samurai sword, then tried to play a saxophone in various unusual ways. Have a look (Belushi shows up at around 1:40):

Zappa came back in 1978 as host, a disastrous performance that would get him banned from the show. The fun part is that Belushi joined the band again for a performance of "St. Alfonzo's Pancake Breakfast" and "Rollo," this time in full Samurai Futaba garb. Belushi shows up just before the 1-minute mark here, playing a bizarro guitar/mic combination:

Zappa later commented in a 1982 interview, when asked if he knew John Belushi: "No, I didn't know him very well, but I am sad that he's gone because the guy was a comedy genius. He was great, he had it, he was a funny guy. They don't build them like that too often. Style-wise there was a big difference between Ackroyd and Belushi; I would rate them as equals. Ackroyd was more intelligent and Belushi was more fall-down-and-hurt-yourself."

In the same interview, when asked if he liked the Blues Brothers, Zappa allowed: "I didn't care for them as a musical act but I did admire the fact that they recorded 'Rubber Biscuit.' Any person who records 'Rubber Biscuit' can't be all bad." If you're unclear on that tune, uh, behold "Rubber Biscuit."