Well, Fleetwood Mac brought in Dave Walker for one LP (Penguin), on which he sang two songs; of these, “Road Runner” stands in my book as one of the group’s all-time lows, with Walker singing in a grunt-sweat-and-strut style more suited to Grand Funk Railroad or Bad Company than anything that Fleetwood Mac were ever about.
On the other hand, “Rocket Cathedrals” off Be-Bop Deluxe’s first LP (Axe Victim), sung by bassist Robert Bryan, is one of my favorites by that band.
I also happen to very much like “In Another Land” on the Stones’ Their Satanic Majesties Request, written and sung by Bill Wyman. I bet some people would list that in the “big mistake” category, though.