Many episodes of LOST have dealt with mysterious recoveries of the characters.
Jack and SarahEdit
After Sarah was rushed to the ER after colliding with Adam Rutherford in a car accident, she was saved by Jack and put into ICU. Jack informed her that her back was broken and that he was going to attempt surgery, but that possible surgery put the likeliness that she would retain any mobility below the waist extremely unlikely. Jack attempted the surgery anyway, and assured Sarah that he was going to fix her.
After he had completed the surgery, he headed to a nearby stadium to run, and twisted his ankle and was aided by a man named Desmond. When Jack explained his current situation with Sarah to Desmond, the question of "What if you did fix her" was raised, to which Jack put it simply as "I didn't" and that if he had it would have been a miracle. Desmond then questions Jack's faith in miracles, to which Jack chuckled at the thought.
Jack would return that day to the hospital where he was shocked to see that Sarah had feeling in her legs, and had miraculously recovered.
John Locke has previously been paralyzed from the waist down for 4 years after his father threw him out of a window ("The Man from Tallahassee") and had to use a wheelchair to get around. After the crash of Oceanic Flight 815, he arrived on the island, with his paralysis cured. Locke could once again walk.
In "Deus Ex Machina", while attempting to gain access to the Hatch with Boone, Locke lost all feelings in his legs, and was unable to feel any pain within them. Locke then had a dream about a Beechcraft crashing on the island, and set out with Boone to find it, determined that the island wanted them to find it. As they traveled, Locke would continuously collapse and lose the ability to walk as they approached even closer. When Locke confessed to Boone that he was paralyzed, the two of them made every attempt to find the Beechcraft and were eventually successful. When Boone arrived inside, he discovered that the only thing present inside was heroin, and that the plane was being flown by drug smugglers. Suddenly, the plane's weight shifted and crashed into the ground with Boone inside. Locke regained the ability to walk, and carried Boone back to camp.
Jin and SunEdit
In "The Whole Truth", Sun realizes that she is pregnant, and is shocked to hear this as she was told by her doctor in Korea that Jin was incapable of having children. Sun assured Jin that she had never been with another man, and Jin was convinced that the event was a miracle. However, in The Glass Ballerina, it is revealed that Sun did, in fact, sleep with Jae Lee. However, it's later revealed that the baby is, in fact, Jin's.
Flashbacks in "S.O.S." show that Rose was diagnosed with an unidentified terminal illness and had only a year to live. When Bernard set out to create an S.O.S. sign in the sand so that a plane flying overhead would see, Rose became concerned as she believes that her terminal illness has gone into remission because of the island. Rose had also spoken with Locke indirectly about miracles, after his legs were crushed by the blast doors. Locke said that Jack had estimated it would be several weeks before he would walk again, to which Rose replied: "But, you and I know it's not going to take that long".