If the killer stepped in the oil next to the car without realizing it, as was 'proven' in the final courtroom scene, there would be footprints leading out of the garage (evidence the police would have had), Ben wouldn't have had to step in it to figure it out, and a lot of the rest of the plot would have been unnecessary.
The viewer is left with the impression that the puddle of oil beneath the Porsche 911 which Matlock stepped into came from the vehicle. The car was pulled all the way into the garage, so there's no way the oil came from the 911; it, as were VW Beetles and Chevrolet Corvairs, was a rear-engined vehicle and its crankcase was a few feet behind where the puddle was. It wasn't possible at all.
At about the 23min mark, LeAnn and Elain are sitting in the amphitheater with open umbrellas. You can hear the raindrop audio and at one point LeAnn peeks out from under her umbrella to check the sky. During this whole time not a single drop of water drips off either umbrella.
The flashback scenes took place circa 1952. The 'Cobrahead' streetlight designs seen in the flashbacks did not come into service until 1957.
In one scene, Ben is seen cooking eggs and sausage. When Leanne enters, she asks how his date with her friend, Elaine went. Ben tells her he ripped his pants and angrily flips his spatula, accidentally soiling Leanne's shirt. In the next shot where Leanne is standing by the table, the stain is gone before Leanne had the chance to wash the stain off.