Don't Forget the Diver - S4-E2
Factual error: While Capt. Square is looking through his binoculars at what he thinks is Mainwaring's platoon marching with their rifles, a 1960's Bedford lorry drives past on the road behind them.
Factual error: In several scenes during the cricket match while the Walmington platoon are bowling/fielding, cars can be seen on the road behind the cricket ground that were made well after the 1940s.
Battle of the Giants! - S4-E14
Factual error: When Mainwaring is driving the van and Jones is in the back suffering from malaria, just after Pike shouts out that Jones has changed colour, if you look out the side window you can see a triangular road sign. These did not come into use until the 1960s.
The King Was in His Counting House - S5-E7
Factual error: Captain Mainwaring is transferring money from his bank to another branch after it has been bombed. He has the money in a basket and is transporting it on a horse and cart when some of the money starts blowing out of the basket. Pike, Wilson, Walker, and Jones are riding bicycles behind him. They stop to pick up the money and just before Jones tells Pike to ride after Mainwaring to tell him about the money, you can see a silver car parked on the left hand grass verge in the direction Mainwaring is heading. This car was obviously made long after World War Two finished.
Factual error: The motorbike and sidecar used by the verger is a post-war 1945 BSA M21 with sidecar. The period is supposed to be 1942.
The Making of Private Pike - S9-E2
Factual error: When Private Pike is on parade and keeps looking at his watch so he is not late for his date at the cinema, the watch is too modern for the period.
Answer: The show is a comedy, this was played for comedic effect and to show that in times of extreme fear, in this case about to be crushed by a steam train, the men had an adrenaline surge strong enough to pedal fast enough.