Continuity mistake: Barry is ordering at the café, and a man with a beard and a man bun walks behind him. An instant cut later, he's already on the other end of the counter. Too fast. Also, the people standing or sitting behind the counter are not continuous between shots.
Continuity mistake: When Young Flash agonises, he is handed a teddy bear whose face is visible. An instant cut later, the teddy bear has turned around. Then, yet a shot later, it's turned again and you can now see his face.
Continuity mistake: When Young Flash is dying, Old holds him in his arms, both bodies next to each other. After Dark Flash vanishes, from the opposite shot of Old Flash he is not holding anyone, just mimicking as if he were. You can see through the gap between his arm and body. A shot later Young Flash reappears.
Continuity mistake: During the Gotham chase, a group of kids get off the school bus, cross the street, and stand in the middle of the road. When Batman launches the puncture spikes, they get off the bus again.
Continuity mistake: When Kara flies upwards holding Barry, she turns her head towards him. From a side angle, her hair is motionless despite the wind and speed. The angle changes to a front shot, and now her hair flutters wildly. (01:36:13)
Continuity mistake: Flash arrives in the past and lands next to the kerb, where he leans on to help him stand up. A shot later, he is several metres away, near the middle of the road.
Continuity mistake: When Flash sees his mum shopping, she is holding a cereal box. From the angle shot from behind, the box is gone.
Continuity mistake: Young Barry is lying half a metre away from the wall. A shot later he is 3 metres away. (00:45:10)
Continuity mistake: Old Barry replies to his dad that "school is good, school is great," and gestures raising his right hand. From a side shot, it's lowered. Then back to a front shot, and he is lowering his hand.
Continuity mistake: When the group enters the Siberian elevator, Kara's arm is bent. A shot later, it hangs down her side.
Continuity mistake: When the Barrys, Kara, and Bruce exit the Siberian lab, they encounter an employee. At first, the top of his hat is wrinkled. A shot later, it's stretched upwards. This swaps back and forth.
Continuity mistake: In Batman's lair, after Barry has been electrocuted, the blood on his nose and mouth suddenly vanishes.
Continuity mistake: Old Barry collapses next to Young Barry, and lies next to his waist. A shot later he lies next to his head.
Continuity mistake: At the lab, when Young Barry collapses, his head lies on a white tile and his hair is all messed. A shot later it lies on a coloured tile and it's not messed.
Continuity mistake: Young Barry forces Old Barry into eating broccoli, which he does in large bites. When Young retrieves the broccoli, it's uneaten.
Continuity mistake: When the movie begins, the way the barista holds his notepad changes between shots. (00:01:07)
Continuity mistake: When Batman and Falcone are pulled up, the case with the virus is open (you can see the bottles). When they are on the bridge, suddenly the case is closed. (00:12:29 - 00:13:00)
Continuity mistake: When Bruce is explaining about time travel issues to the Barries, the amount of sauce on the spaghetti changes between shots. The spaghetti in the bowl shifts as well, most noticeably when there's some right up on the lip of the bowl close to Bruce, which then disappears from another angle.
Continuity mistake: During the post-credits scene, after Arthur falls on the puddle, there's a leaf on the pavement in front. When he turns around begging for beer, the leaf disappears, only to reappear a shot later.
Answer: Some possible reasons gleaned from the Internet: Filmmakers wanted a different vision of Supergirl to reflect a darker, grittier personality and to show her refugee-like experience. Her dark short hair symbolizes her harsher journey from her home world. Also, it gives her a more androgynous look, reflecting both her feminine and masculine traits. Adopting a radically different look for an established character also shakes up viewers' expectations and generates more publicity and interest in the film.
raywest ★