Continuity mistake: While Wizard is commenting on the bit about 'f**s' in California, Travis who was still getting his order from the Belmore clerk is suddenly already midway towards them across the room. (00:45:10)
Continuity mistake: Betsy opens the door of the taxi and tells Travis that she's already got the record. Her handbag is held up high enough to be in front of her blouse's buttons in the first shot, but not visible at all in the next close-up. (00:36:30)
Continuity mistake: Travis and Betsy are having their snack during her break. During the conversation there are several instances when vehicles you can spot through the window behind De Niro do not move to Cybil Shepherd's side, or vice versa suddenly show up behind her appearing out of nothing. For instance when he says he says "quite a few problems" the orange car does not travel in between shots, and then a taxi appears behind her. Then there's a bus behind him which is suddenly gone between shots, etc. (00:25:00)
Continuity mistake: During the chat the protagonist and Cybil Shepherd have at the diner, the passersby outside often walk back and forth. Easily recognizable for instance a redhead with a white shirt, blue skirt and a red ribbon tie, a woman with a blue tee and grey hair, another one in a pink tailleur and straw hat, and many more. (00:23:30)
Continuity mistake: In the last part of the 'stool pigeon' bit between Cybill Shepherd and Albert Brooks, the paper sheet rolled in the typewriter has moved without Tom operating the device at all between shots. His hands also change position between shots (for instance, he points his finger with both hands joined at first, but at the cut just one is raised). (00:19:50)
Continuity mistake: When Betsy pranks Tom with the paper cup, she 'spills' it with the right hand in the front view, and holds it in her left hand in the reverse shot. She then lifts it well above shoulder level and turns it upside down to show that it is empty, but in the next shot it is at the same height and angle as before. (00:18:25)
Continuity mistake: In the first scene with the fellow cabbies at the diners, Wizard introduces Doughboy and Charlie T to Travis. Charlie in the very first shot has his toothpick on the right corner of the mouth, but in the close-up it is all the way to the left. (00:16:00)
Continuity mistake: When Betsy is pointing the taxi driver outside to Albert Brooks, both her hands and his change position in the next shot, when he asks if the cabbie stalker is bothering her. (00:12:55)
Continuity mistake: Travis is at the concession stand of the theater and has all his candy bars on the counter. Even after he draws the popcorn in, the bars are off the mag the dismissive cashier is reading. She reaches for the cheap cola she somehow manages to pour, and when she puts it with Travis' stuff the magazine is open at a different page and the candy is on top of it. (00:09:25)
Continuity mistake: After applying for his job, Travis walks towards the exit of the garage, jacket unzipped. In the next shot, the passersby change and his jacket is zipped 1/3 way. (00:04:35)
Continuity mistake: The guns Travis purchases from Easy Andy are (regardless of what Andy calls them), from left to right, a Smith and Wesson Model 29, a Smith and Wesson Model 36, a Smith and Wesson Escort, an Astra Constable. In the overhead shots at the gun range when Travis goes to practice immediately after, some of the guns change: the first 44 Magnum is the same, but the 38 becomes a Colt Detective Special, the S&W Escort a Galesi-Brescia Brevetto 5, the Astra a Walther PPK (which is what Andy called it earlier).
Continuity mistake: The close-up of the taxi fare meter during the scene with Martin Scorsese's cameo has lighting inconsistent with the rest of the scene. (00:41:00)
Continuity mistake: When the store owner beats the carcass of the robber, Travis' pistol is either to the right of the box on the counter or to the left, together with the gun of the robber. (01:09:30)
Revealing mistake: During De Niro's voiceover in the "Dear father and mother" letter, the camera pans towards the stage where Senator Charles Palantine is giving his speech. As De Niro mentions the anniversary, a dude in the crowd spots the camera and waves hi to it, says something like "Wonder what's up?" to someone next to him in the crowd and leans to his left trying to stay in frame longer. (01:12:05)
Continuity mistake: Travis Bickle is preparing for his mission. He puts the flowers in the sink and burns them, he cuts one sleeve from his shirt so he can accommodate a special retractable holster, and then...then he is writing a note for Iris, wearing a shirt with both sleeves, in a room still full of wilted flowers. (01:33:00)
Character mistake: The "Taxi Driver Hero to Recover" newspaper clipping misspells "Pittsburgh" at the end of the first paragraph as "Pittsburg." (01:46:55)
Continuity mistake: The breakfast scene ends with Jodie Foster having a bunch more toast on her plate than at the start. (01:26:40 - 01:29:20)
Continuity mistake: When Travis joins Easy Andy in the cab, cars and people in the street behind him change in the reverse angle. (00:54:00)
Continuity mistake: In the close-up of the stabbing, Murray Moston's hand looks different compared to the previous shot. (01:41:20)
Other mistake: Travis built a special arm holster for a gun that allows him to pull a gun easily, popping it in hand while his arm is already outstretched. It's the weapon he prepared for in the iconic "You talkin' to me?" scene, and practices handshaking with it. We see a scene when he prepares to go kill the Senator cutting the sleeve of the shirt off. And yet, when the actual attempted hit is shown, Travis has his hand (the gun hand) inside his jacket, alerting security. (01:36:30)
Answer: It helps him create a tolerance for pain. As we see, he certainly gets a lot of that dished out to him.