A policeman pulls a wad of crumpled paper from a waste basket and opens it up. The paper he hands to Barnaby is clearly not the same piece of paper.
When Cavendish is showing the detectives his collection of German military memorabilia, he produces a medal which he refers to as the 'Iron Cross First Class'. As can be seen from the ribbon, it is in fact an even higher award, the Knight's Cross, with the further distinction of oak leaves. No serious collector (as Cavendish is apparently supposed to be) would make such an elementary mistake - it would be almost as bad as getting the denomination wrong on a coin or a stamp.
Since RLB daggers were issued to members of the German air raid warning services, it is unlikely the man who said it was used at El Alamein was right.
When explaining why she is telling Barnaby that Steven Cavendish is lying, Mrs. Cooper quotes a verse from the book of Philippians but attributes the words to Jesus instead of Paul.