Jonny's 1911 pistol is single action. It was not cocked, so pulling the trigger in that condition would not fire it. The magazine holds 7 rounds. After firing 7 shots it can be seen that the slide locked open as it should when empty. Then Sadie fires an eighth shot.
When Jake has made the students at the door go away and returns to the room where Sadie is, he walks behind Johnny. It should be easy for him to overcome Johnny and take his gun.
When Bill is staking out Colonel Walker's house at night, he looks at his watch to check the time. His watch says "Quartz" on the face. Quartz movement watches were not available until late 1969, when Seiko introduced the Astron.
The titles say it is 1963. When Jake and Bill load their trunk, their car has a black 1960 license plate instead of the white 1963 plate. The other cars also have black plates and no front plate. Texas required a front plate.
When Jake is clearing the apartment he puts rubbish into a black plastic bin bag. Although green coloured waste bags were invented in the 1950's in Canada for commercial use, it was not until the late 1960's they were manufactured for domestic use. Even then they were green and not black.
Modern day smoke detectors visible in the hallway ceiling when Jake is being interviewed by Deputy Stephens at Parkland Hospital.
At 15:16, you can see a telephone pole with a modern era cable trunk amp. This didn't come out until the late 70's. This has to do with cable TV.