Other mistake: In one of the opening scenes where the boys ride to the baseball game, the fishermen they pass make a rude comment towards them. In response, you hear Joe ring his bicycle bell and see them flip the men off. The bicycle has no bell.
Continuity mistake: When Rebecca Wenteworth is walking towards the baseball diamond, she is looking behind her and waving. When she gets hit in the head with the ball, it's on the wrong side of the face. She should have been hit on the left side of the face.
Continuity mistake: In the Christmas Pageant scenes, Miss Leavey is seen pulling down on the rope that raises or lowers the kid playing the angel. By pulling down on the rope, the kid should go higher. But when Miss Leavey pulls down on the rope, the kid is lowered as well.
Continuity mistake: As the bus heads down the hill towards the lake we see Simon with his glasses on - then there is a close-up of him without his glasses, then as the bus hits the water he's wearing them again and they fly off and hit the floor.
Other mistake: In the second baseball scene, Joe is sitting on the bench next to one character with the number 8 and Simon, who is on Joe's other side is also number 8. Two players on the same team would not be the same number.
Continuity mistake: As the bus is leaving the church, you see Joe get on the bus and the door close. A few shots later there is a shot of Simon and the bus door is open again.
Continuity mistake: In the scene where Simon goes in the church to retrieve his baseball cards he climbs on the file cabinets with a flash light. He places the flashlight on top of the card box which is shining to the right. The flashlight falls behind the box, still facing right. The next shot is the back of his head, then a shot of his puzzled facial expression. The next shot is of the flashlight facing left on the baseball. (00:48:25)
Answer: I don't know if it was ever stated what Simon died of, but Simon - against the odds - lived much longer than the doctor thought he would. Simon continued to "outgrow his body" (internal organs became too large for his tiny body). It can be assumed that Simon contracted pneumonia or hypothermia from being in the frigid water when he rescued the children from the submerged bus. I think Simon - expected to die at a very young age - was staying alive until he could realise his purpose or "God's Plan" for him. To "believers", Simon was "made small" and practiced holding his breath underwater (seemingly for no apparent reason) so that he'd be able to save the kids on the bus... then, having fulfilled "his purpose", was called "home."
KeyZOid