Factual error: The only ways Tim/Ben could have died are by heart failure and/or respiratory paralysis. Both of which mean that the toxins reached the heart. If they hadn't, then he would only have had a paralyzed right arm and been in extreme pain. The logic of this movie implies that either his heart was unaffected by the toxins and he still somehow died or his heart was affected by the toxins and they decided to transplant it anyways.
Continuity mistake: When Ben is lying in the bath dying, he pulls down the shower curtain covering the note he has left on the floor for the doctors. When the scene changes then changes back to him in the bath, the shower curtain's position has moved. (01:49:00)
Other mistake: Ben brings Emily home from the hospital and Duke is in the back seat. When Ben stops and puts the car in park you can see in the background that the car is still moving.
Continuity mistake: When Will Smith goes in the car with Emily, the seat headrest is down, but just before Emily kisses Will the headrest is up.
Other mistake: Ben goes to meet a friend in Social Services. On the door, it says "Sevices."
Other mistake: When Ben is in the car with Emily, they pull to a stop and Ben shifts the car into PARK. However, the car is still visibly moving.
Visible crew/equipment: When Ben is driving his car along with his wife, you can see a crew member's face reflected on the glass window of the car.
Answer: In the course of the film it's revealed that Ben has done extensive homework and taken drastic steps to find the right person, i.e. a "righteous good person" to give them what he has left to offer. Therefore, he would have to find someone with his own rare blood type AND needed a heart...which he eventually did.
CCARNI