When Walker goes to Lynne's apartment, the array of magazines on her coffee table shifts wildly from shot to shot.
When Walker shoots up the bed, the holes shown are way larger than would come from a handgun, except maybe if they were contact shots, which they were not. None are at the angle from which he is shooting, and there's already a hole in the bed where he takes his last shot before he does.
When Walker has a flashback about shooting up the bed, it's obvious Lee Marvin simulates the recoil of the gun before it fires.
When Walker goes to turn on the bedside lamp in Chris's darkened bedroom, the "outside" lights go off out of order and the "inside" lights come up before the lamp's bulb comes on.
(at around 1h) A sniper is seen clearly aiming away from his target; however, he kills the man with the first shot.
When Walker is taking his "test drive" and smashing up the car, the sound of car wheels squealing on asphalt is heard, but he's driving on dirt.
After Chris leaves Walker in her apartment, Reese is shown standing and staring through a large plate glass window as though he is looking outside, but the reflection of a red camera light can be seen in the glass.
When Big John sits in the car with Walker at the car lot, a large studio light is reflected on the side window of the windshield.