In the VN it appears at least once in every route.
Does the actual translation from Japanese have this line, or is it an inside joke?
From the VN(and original anime line):
Shirou: People die when they're killed(giving back Avalon to Saber)
Tohsaka: Look Emiya-kun. People normally die when they are killed. Your wound should be fatal, you know?
Kotomine: That is exactly right, Emiya Shirou. It is the law of nature for people to die from wounds. But you cannot kill what has not yet come into existence.
HF (good end)
Tohsaka: He can go get surgery at hospitals, drink cold medicines, and die when he's killed.