Entering this in the category of truly obscure and only found this out 100% by a chance google a couple of weeks a go. Can't even remember why I googled this song.
George Harrison ended up playing on a song on a 1973 Cheech and Chong album. "Basketball Jones featuring Tyrone Shoelaces" which was later remade with Barry White and Chris Rock but the original started in the studio with just Cheech and Chong playing/singing. Sort of went all-star studio session from there.
Wiki - Some notes on the recording of the track, taken from the booklet accompanying Where There's Smoke There's Cheech & Chong, read:
Cheech sings, and Tommy plays piano—that's all it was at first. In Cheech's words, "George Harrison and those guys were in the next studio recording, and so Lou [Adler] just ran over there and played [it for him]. They made up the track right on the spot." "That was a wild session," Lou Adler recalls. "I probably called Carole [King] and told her to come down, but with Harrison and [Klaus] Voorman—I didn't call and say come in and play. Everyone happened to be in the A&M studios at that particular time, doing different projects. It was spilling out of the studio into the corridors."
So in an impromptu session it went from Cheech singing and Chong on piano to having names like George Harrison on guitar, Carole King on piano, Billy Preston on organ and Ronnie Spector and Michelle Phillips singing backup!