Mel Blanc (1961, Car Accident)
In 1961, Mel Blanc was involved in a near fatal car accident when a drunk driver hit his car. "Hit head-on, Blanc suffered a triple skull fracture that left him in a coma for three weeks, along with fractures of both legs and the pelvis". His voices for Warner Brother's were ghosted until he returned. He died in 1989 after being brought into the hospital for some health issues, staying overnight, and falling off the bed because they didn't put up the rails up on the sides.
Robin Williams (Late 70s to early 80s, Drugs and Alcohol; Mid 2000s, Alcoholism; Late 2000s, Heart problems)
Robin Williams was addicted to cocaine during said period, and perhaps a few other things, and was a frequent partier alongside his friend John Belushi. Belushi's death prompted Williams to quit drugs.
In 2006, Williams went into rehab because he had relapsed into drinking heavily again after 20 years of sobriety.
In 2009, Williams was checked into a hospital for heart problems. He underwent surgery to replace his aortic valve, and the surgery was successful.
Joe Piscopo (1981 to 1982, Thyroid Cancer)
Self explanatory.
Bob Dylan (1966, Motorcycle Accident)
On July 29, 1966, Bob Dylan got into a motorcycle accident near his upstate New York home. Reports at the time were that it was serious, but report since then seem to say it was overstated and was only a minor accident, but Dylan used it as an excuse to drop out of music for a while so people would leave him alone. It would be easy to see an alternate scenario, though, where it could have been worse, and fatally worse.
John Lennon (1967, Possibly Falling Off Roof during Acid Trip; Late 60s to 1970s, Heroine)
You could probably do quite a few possibilities on the Beatles, and probably even more on John Lennon than I'm pointing out here (he led a rock n' roll life, and did sex, drugs, rock and roll, and getting into fights and all of that). Anything Lennon did, the other Beatles often did too, but I'll focus on two specific instances. In 1967, George Martin, not knowing the Beatles were doing LSD nor that Lennon was on an acid trip at the moment, took Lennon up to the roof of Abbey road for some fresh air (the outside had Beatles fans, so the only place they could get outside for a bit was the roof). In the OTL, it was a starry night and Lennon sat on the edge of the roof looking up at the stars. In an alternate timeline, given his state, maybe he could have lost his grip and fallen.
Another one was the fact that he was doing Heroine for a number of years on and off (and whether he was addicted or was addicted at any period or periods is debateable). Yoko has said what saved her and John from addiction in the 70s was that they had a cheap drug dealer who mixed up the heroine with baby powder, so they were getting weak doses and could more easily kick it unlike many musicians and celebrities they knew who got pure doses and ended up addicted and dying. She also said the fact that they were afraid of needles and didn't use them helped. So for want of a drug dealer, they avoided overdosing or otherwise having their health and mental state deteriorate due to addiction.