When the trebuchet crashed, a piece of shrapnel stuck into Locke's leg causing a painful injury.  Later he has trouble walking and fears he is losing use of legs again.

Locke is a kidney donor. He gave his kidney to Anthony Cooper, the man who claims to be his father. In a flashback, we see Locke bleeding from his back while attempting to confront his father.

Sawyer said his uncle had a brain tumor. Jack tries to figure out why Sawyer is having headaches and asks him if he has an STD. Jack is egging Sawyer on, and Sawyer actually has hyperopia, far-sightedness. Sayid constructs groovy glasses for Sawyer, which are presumably lost in the raft explosion.

 

Boone climbs into the Beechcraft, tries to send a transmission over the radio, and the plane crashes to the ground. He is seriously injured.

 

Locke's mother, Emily, is schizophrenic and was institutionalized at least five times by 1995 at the Santa Rosa Mental Health Institute (assuming her admissions papers are correct). She was both self and doctor admitted.

 

Locke's legs were failing, so Boone carried him in the direction Locke pointed. When they stopped to rest, Boone told him that his mother wasn't around much when he was a kid. He needed to take it out on someone. Theresa was his nanny and he used to lay in bed all day pushing the intercom to make her come up and down the stairs for him. One day when he was six, she took a bad step, fell down the stairs and broke her neck. Excerpted from Locke's Prophetic Dream entry.