Black Sabbath with Ian Gillan (1983-'84)
Against all odds, Black Sabbath briefly grew stronger after Ozzy Osbourne was pushed out in 1979. The last few Ozzy albums were highly disappointing, and new singer Ronnie James Dio infused the band with incredible new energy. But he only lasted two albums, and by 1983 they were once again in search of a new singer. Deep Purple had folded by this point, so their vocalist, Ian Gillan, needed a gig. It seemed like a good idea on paper. There's a lot of overlap in the Black Sabbath and Deep Purple fan communities, but their lone album together, Born Again, just didn't quite click. (It also has the ugliest album cover in rock history.) The supporting tour was such a fiasco it inspired Spinal Tap the following year. They built a Stonehenge stage set that was too big for almost every venue they played, and Gillan walked when the tour was over and reformed Deep Purple. He remains there to this day.
Black Sabbath limped along with unknown singer Tony Martin, ultimately reforming with both Dio and Ozzy at various points in the future. The world has largely forgotten Ian Gillian's brief tenure in the band, but it was a good cautionary tale for Velvet Revolver and Scott Weiland. You can't just smash together two great bands and expect it to work. People want Ian Gillan singing Deep Purple songs, and they want Ozzy Osbourne fronting Black Sabbath.