34. The war veteran

The year is 1971. An old man limps slowly into a war claims office and requests compensation for injuries suffered in battle more than half a century ago during the First World War.

He produces an old, yellowed letter dated November 1921 which reads:

I hereby confirm that Charles William York fought gallantly in the First World War under my command and sustained severe injuries in battle.

—Signed General John J. Pershing

The office secretary immediately declares the letter to be a fake. How did she know?