Episode #4.9 - S4-E9
Revealing mistake: In a scene in the mess, Sicknote walks in front of the television and there are grey stickers stuck to the screen to hide the production spotlights.
Episode #4.7 - S4-E7
Factual error: In this episode Blackwall Station is shown as a London Underground tube station. Blackwall Station is a Docklands Light Rail (DLR) station. Also look at the destination sign on the front of the train, which reads 'THAMES LINE', No such line/station exists on the London Underground.