Other mistake: Liam Neeson jumps into ice water to rescue his brother. He then painfully pulls his brother into the cab of the truck. Inside, he revives his brother after which they have a lengthy conversation. Liam's Mike McCann never starts the truck engine or turns the heater up, even though he and his brother are soaked to the skin and the brother is coughing up ice water from his lungs. In the next scene, both appear dry, in the same clothing, with no signs of hypothermia.
Factual error: The Black Watch sergeant major wears a crown over three chevrons, which was the correct rank badge for a company sergeant major until 1915. However, this is at least 1916 (as conscription has started and the men are wearing steel helmets, both introduced in 1916). CSMs had been redesignated as warrant officers in 1915 and now wore a crown on the lower sleeves instead.
Continuity mistake: Rudy looks at the candy cane, which is broken off at the handle/arc. Rudy sucks on the candy cane while he talks to his mum on the phone. When Rudy removes the candy cane from his mouth, the curved part is longer than when he first puts it in his mouth. (00:08:40)
Factual error: The movie largely takes place in California during the 1990s. California's mandatory seat belt law went into effect 1/01/86. When Mickey was driving Kyd to the cemetery, they were not wearing seat belts. Also, Kyd was 8 years old at the time. Although legally permitted to ride in the front, it is advised that kids sit in the back seat for safety. Considering Lyla was recently killed when a driver ran a stop sign, Mickey should have been more concerned for Kyd's safety as a passenger. (01:12:58 - 01:15:05)
Revealing mistake: The "bridge" within the crime scene tape is obviously a prop. It should be a continuation of the road, but it is tilted to the left and is more vertical (around 45°). The bridge isn't even very high. When Byron and others go under the bridge to look at the dead girl, there is only a few feet above their heads (clearance under the bridge). (00:04:54)
Factual error: Morse code is supposedly being transmitted by the blinking street lights. They supposedly spell "For 3veryth5h1ng that 7iv3s is holy." I know Morse code. It was just a random scattering of letters. (01:00:05)
Factual error: Dylan (Scott Eastwood) was in a jail cell yet had a bottle of prescription pills in his pocket. The pills would have been confiscated before he was put in the cell and administered to him on schedule by staff/nurse. (00:28:10)