As sad as Phil’s untimely demise was (and I most certainly agree that it was), it seems less like most of the other entries in this thread, and more like the number of celebrities who died of an overdose or at the hand of a sociopath - the list of which is ridiculously long.
I was going to mention Dana Plato then discovered I already had, almost four years ago.
Is this thread the record for time in the grave?
I was really disappointed to see Frank Thornton (Captain Peacock from Are You Being Served?) as Maggie Smith’s butler in Gosford Park- a non-speaking role that had 10 seconds of screen time.
I read recently that the reason some stars did Love Boats and guest appearances on sitcoms you’d think would be beneath them had less to do with money (though it’s still an easy several grand) than for SAG and other professional membership criteria to keep their insurance and memberships current. Made sense.
I also remember an article by Vic Tayback’s son that was interesting. People thought his father was penniless because just before he died he was doing dinner theater in a really provincial town (like the ones I live in) and figured he must need the money. His son said (paraphrasing) “Dad was a multimillionaire who never had to work again. He loved the applause, he loved being treated like a superstar when he went to these places, he loved acting- and who was gonna cast my dad as Fagin [or whoever] on Broadway?” Seemed reasonable.
Bob Crane went from star of the classic “Hogan’s Heroes” to being blugeoned to death in an unsolved murder (probably related to his porn collection).
Wanna see somethin’ really scary?
At the bottom is a very non-work safe link to Bob Crane’s son’s site which is dedicated to his father and selling “memorabilia” thereof. It’s one of those “I really really don’t want to think of doing this for my dead dad” sites, but I guess if your birthright was $300 and a warehouse full of dad’s porn types then this is making lemonade.
Link (actually the homepage is work-safe enough, it’s the “merchandise” and other links).
Yeah, I wouldn’t even classify him with people who OD’d. The “sad ends” in the rest of this thread are mainly the result of peoples’ inabilities to manage their careers or personal lives. Hartman may have made a poor choice in spouses (I say ‘may’ because Brynn could have been a much different person when they met), but we don’t know what he might have done to try to help the situation. So his story is simply sad, through no discernable fault of his own.
(They’re showing an SNL tribute to him on I think Comedy Central right now. Clinton at McDonald’s. That was a classic. “Warlords!”)
Barry Cowsill of the famly musical group The Cowsills became a drug addict and homeless and was living on the streets in New Orleans. He drowned during Katrina. The Cowsills were the group the TV show The Partridge Family was modeled after.
StG
It doesn’t compare to some of these ends, but I was greatly saddened when I saw Pat Morita appear in a couple of really lame Nissan commercials. Adding insult to injury, he was second fiddle to a dog, and all he did was say “Dogs love trucks.” 
Just recently, I’ve been seeing commercials for a roofing company that have Evel Knievel doing voice-over while stock footage of a failed jump plays on the screen.
Margaux Hemingway
Captain Beefheart
Hervé Villechaize
Gary Coleman
Wasn’t John Travolta’s career pretty much reduced to movies about talking babies until Quentin Tarantino cast him in Pulp Fiction?
Adam West spent years typecast as Batman in the '60s TV series, and even now he’s best known as the quasi-insane Mayor Adam West in Family Guy- in effect, playing a demented version of himself…
Redd Foxx? Hold your horses, everyone…
He might have had very little money when he died, but you can’t take it with you. He didn’t piss his money away on himself so much as overindulge needy “friends”. And the show that he starred on just before he died had both respectable critical aclaim and reasonable ratings (for a new show), if memory serves; his presence was too big a part of the show to do without, so it was cancelled. Imagine: right before he dies, he reads in the paper that the critics think his show is off to a good start, and ratings are good. That’s what actors live for. He was not a young man. His life seemed happy. What’s the sad part?