oh, and he's not my favorite singer, but put Scott Weiland down for driving at night without headlights.

The Clash selling-out with Combat Rock.

Definitely, the Beatles and Yoko Ono...

IMHO Combat Rock was not at all a mistake although it's far from perfect.
Cut The Crap, well, that's another story.

I'll add some ones I thought of for other bands/artists:

The Beach Boys:
1) Letting Bruce Johnston write and sing his own material on the albums.

2) Letting Brian Wilson come back on tour in the mid-70's (I really like Brian as an artist, but he clearly was not ready for this).

3) Letting Dennis Wilson perform "You Are So Beautiful" live. He actually had great original material, so why he did such a schmaltzy piece of crap like that is a huge mystery.

4) Doing the re-make of "California Girls" with David Lee Roth (and for that matter any re-make they did of this song).

5) Doing a cover of "Wipe-Out" as a collaboration with The Fat Boys.

6) Letting Mike Love go out on recent tours with a bunch of unknown musicians and calling it "The Beach Boys".

The Who:

1) Participating in Ken Russell's film version of Tommy.

2) Not calling it a day after Keith Moon died.

3) 1982 tour (Although I saw them at Shea Stadium and I was thrilled to see them live, this wasn't the circumstance I hoped for).

4) 1989 tour (Has its moments, but Townshend couldn't play electric because he suffered tinnitus, so it looks to me like they were forced to do this tour under duress. Maybe not a mistake THEY were responsible for, but nevertheless something that should have been avoided).

5) The aforementioned Townshend mishap (ITA, and it's a classic example of "When bad things happen to good people").

6) Pete also made a huge faux pas when he lashed out at the families of the victims of the Cinncinatti tragedy in a Rolling Stone interview in 1980.

7) Not calling it a day after John Entwistle died.