There is no such thing as luck--even the mostly unlikely winner put himself in the position by signing the fight and climbing in the ring. That notwithstanding, Moorer winning a close and controversial decision when Holyfield was having a heart attack and organ failure will always be as good an answer as any.
I think luck is overused word in boxing because it still takes skill to land a one punch knockout, for example. However, one fight where luck won a boxer the fight is the Martin vs Glazkov fight. Charles Martin won a vacant belt against Glazkov because Glazkov suffered a knee injury in the ring. Martin hadn’t really done anything in the fight or before the fight (wins) to earn the win and then later went on to get a huge payday against AJ for his belt. Glazkov never fought again, but to be honest, he was also lucky to get a vacant title shot because judges loved to give him wins for fights he lost (Steve Cunningham and Malik Scott).
Holyfield has another going for him in the fan man fight. I believe Bowe would have won (was leading on the cards) if not for giving Holyfield a rest so I considered Holyfield lucky there.
For me there are no lucky punches, but there can be particular situations that favor a boxer. For example I think Cotto would never have won against Martinez at middleweight if the Argentine hadn't had bad knees. And Cintrón would never have received a draw against Martinez if the referee hadn't mistaken a legitimate punch for a headbutt and then the judges hadn't ruled Martinez's clear victory a draw.
I can’t see how any boxer proven to have had a heart attack during a bout would ever be licensed to fight again?
Horn vs Zerafa Horn would have got finished if the ref didn't intervene. Navarette vs Wilson Davis vs Roach