Revealing mistake: The crew rigged the explosion to start from under the car and not from the lit match which isn't thrown anywhere near where the explosion originates. (01:25:30)
Revealing mistake: As she speaks to the cashier, most of the birds perching in the cage behind and to the right of Melanie are fake. The perches sway but the birds don't move. (00:02:55)
Revealing mistake: A red headed kid in a blue shirt gets pecked on the cheek by a blackbird as he runs down the street. If you look closely at the bird before he touches the kid the blood he smears on the child's cheek is in a sack hanging off of his upper beak. (01:14:30)
Revealing mistake: Melanie has no problem taking the letter for Mitch out of her pocketbook as she places it on top of the bird cage. The crew put adhesive tape on the back of it, so she can push on the letter with her thumb to keep it in place for the close-up. (00:10:20)
Revealing mistake: In virtually all the outdoor attack scenes (but most noticeably in the phone booth sequence), the rotoscope seagulls and crows are proportioned about 150% to 200% of their natural size, about the size of turkeys.
Revealing mistake: When hundreds of sparrows pour out of the fireplace, filling the house, they do not cast shadows on the furniture, walls or people.
Revealing mistake: As Melanie walks up the stairs she's pointing the flashlight out in front of her but it's able to illuminate her face like a spot light. (01:46:50)
Revealing mistake: A fake bird, unable to fly straight, swoops down as Mitch saves Melanie from the phone booth. (01:27:45)
Revealing mistake: The cameraman loses focus on the uptight mom as she asks Melanie where she came from. In the next shot she is sharp as a tack. (01:28:50)
Revealing mistake: If you watch closely as she sets sail Melanie's boat nudges the boat that the camera is on causing the film to jiggle slightly as she passes by. (00:20:35)
Revealing mistake: When Mitch runs to get the hammer and nails the camera follows him around the corner of the kitchen door. Watch very closely as the door on the far right of the screen moves when the camera hits it. The film also jiggles as this happens. (01:43:40)
Revealing mistake: When they show the perspective of being in Melanie's driver's seat looking out the windshield we don't see Melanie's legs as you would expect, but a mechanism that allows someone to steer from the passenger seat so the camera has an unobstructed view of the driver's seat. (00:11:05)
Answer: The movie takes place in the 1960s, so presumably gas pumps worked a bit differently back then. There may not have been automatic shut-offs, or if there was, it was faulty.
The automatic shut off is in the handle of the hose. When a person grips the handle they have to lift a levered switch to allow gas flow; the lever is spring loaded so that if it is released the gas will stop flowing. I have looked at older antique gas pumps and they are all the same; even the visible glass gravity flow ones. If that hose was dropped the the gas should have stopped flowing.