Other mistake: When Travis is negotiating with Sport for Iris' services, an off-screen crew member can be heard saying, "but no rough stuff" just before Sport says it.(01:17:00)
Travis Bickle: [Into the mirror.] You talkin' to me? You talkin' to me? You talkin' to me? Then who the hell else are you talkin' to? You talkin' to me? Well, I'm the only one here. Who the f**k do you think you're talking to? Oh, yeah? Okay! [whips out gun.] Huh?
Trivia: Director Martin Scorsese makes more then just a cameo in this film, he's the passenger that sits with Travis talking about how he's going to kill his wife for cheating on him with a black man. He's credited as "Man Watching Silhouette".
Answer: It helps him create a tolerance for pain. As we see, he certainly gets a lot of that dished out to him.