When Clive first visits his Aunt's house, and the camera pans to the wall, where the animal heads will appear. You can clearly see a lighter patch of wallpaper where one of the heads will later be superimposed.
When in 1919 Candy receives the letter telling him that the whereabouts of prisoner-of-war Schuldorff there's a shot of the letter saying that camp is called 'Hardleigh'. However in voice-over Candy says 'Hardwick'.
Candy's scar is clearly visible on his upper lip when he is in his room at the convalescent hospital, but is obscured by his newly grown mustache moments later when he goes to congratulate Kretschmar-Schuldorff.
While waiting for his car to be repaired Clive Candy receives a message and states "Murdoch, the war is over. The Germans have accepted the terms of the armistice; hostilities cease at 10 O'clock." WWI famously ended on the 11th hour of the 11th day of the 11th month, or 11:00 (not 10:00) AM on 11 November, 1918. It should be noted that France is in the Central European Time Zone one hour ahead of England in Western European Zone (Greenwich Time), so if you were in England at the time (Candy was not), it would have been 10 AM.
The notice Candy places in The Times about Murdoch's death states he died in a bombing raid in October, 1940. But when Candy is shown on the cover of Picture Post magazine sometime afterward, the magazine is dated September 21, 1940.