Revealing mistake: In the church, none of the infected that Jim sees are wearing anything on their heads. They then chase him and are set alight by Jim's rescuers. When we see the infected running and burning, the fireproof hoods the actors are wearing are visible.
Revealing mistake: When the camera pans up to show Manchester on fire, flashing lights are just visible on the horizon in the middle of the road. This is the police keeping the traffic back so the scene could be filmed.
Revealing mistake: After the soldiers execute their spiritual comrade, Jim takes cover in the field of dead bodies nearby as the executioners run off to search for him. The body right in front of Jim is breathing, as his chest is moving up and down.
Revealing mistake: When Jim is walking across the Westminster bridge and shouts "Hello?" for the first time, in the next shot you can see a blob on the bridge (right of picture). This is all that's left of a car going across the bridge, that wasn't painted out completely.
Revealing mistake: After Mr. Bridges and his daughter are killed, Selena asks Jim if he was bitten. When he says "no", Mr. Bridges can be seen moving slightly, who is lying on the floor in front of Jim.