Continuity mistake: When Gene Hunt pushes Sam Tyler up against the filing cabinet, he has a cigarette in his mouth. When the shot cuts back to Gene Hunt's face, a split second later, the cigarette is gone.
Revealing mistake: When in the mortuary looking at the female murder victim's body, watch as Sam Tyler says "cord, no other attack marks". When the camera cuts to the corpse at this very moment, you can briefly see the "corpse" swallowing.