Deliberate mistake: David is shown to have an illegal weapon in his home even though gun control is quite strict in the U.K. Sadly, this is quite a common practice by returning war veterans who feel safer with a gun in the house, as they often suffer from PTSD, as David does.
Deliberate mistake: As Chief Vickery, in present time, patrols Wind Gap in his car he passes by the barbershop. From his POV, through the car window, we see four men sitting outside. The same men were also seen in Episode 01, but from young Camille's POV as she and her sister pass by the barbershop in 1992. The show hid a load of easter eggs and signs throughout all episodes. They would hardly make such a mistake. More likely, the repeat was to underline Vickery's backward, stepfordian attitude.