Jimmy Page - Stupidest thing he did was get arrested for smoking a cigarette on an airplane. IMHO, he hasn't done anything musically stupid (I'm sure someone's gonna disagree to that, particularly Plant for the Coverdale stint). Wait. Shit. I take that back: he allowed "In The Evening" to be on the otherwise perfect "In Through The Outdoor". Before that was "Hats Off to Roy Harper".. And if he had anything to do with the horrid "Song Remains the Same" movie debacle.. There, that's it. Any other anomolies in the Zep catalogue are really just things that didn't make the timeless category..
Tony Iommi - Cutting the Seventh Star album under the moniker "Black Sabbath Featuring Tony Iommi". Tony was the only original member of the band at the time.
Randy Rhoads - Not really a favorite of mine and that's his fault for somehow allowing a buffoon to ruin his sound in the studio.
Kirk Hammet - the wah pedal
James Hetfield - don't even get me started.
George Harrison - Marrying Patty, the catalyst for "Layla", THE most torturous, agonizing warhorse in the history of rock (right up there with the Whitney Houston "I will always love you" cover in pop)
Paul McCartney - seeping his schmaltz goo all over the Beatle albums. Most often not a bad thing (e.g. "Abbey Road") but once in awhile so gelatinous that it steps on the toes of Lennon's work (e.g. Sgt. Pepper).
Jimi Hendrix - "And the Gods Made Love"
Alex Lifeson - Apparently getting castrated after the first Rush album.
Dave Mustaine - whatever he did to his arm, either overkilling the technique or improper technique.
Vivian Campbell - Leaving Dio to play with the Leppard girls.
Steve Vai - releasing or taking part in anything other than "Tobacco Road", "Damn Good", or "The Audience is Listening".
Cy