B-17, Tondelayo, was barraged by flak from Nazi antiaircraft guns, the gas tanks were hit 11 times by 20 mm shells without touching off an explosion.

The crew wanted the shells as a souvenir of unbelievable luck, so the rounds were sent to the armory dept to get defused... 

when the armorers opened each of those shells, they found NO explosive charge. They were harmless.

But one shell contained a carefully rolled piece of paper. On it was a scrawl in Czech.

The Intelligence people scoured the base for a anyone who could read Czech. Eventually they found one to decipher the note.

Translated, the note read: “This is all we can do for you now. Using Jewish slave labor is never a good idea.”

A member of the Czech underground, working in a Nazi munitions factory, had deliberately omitted the explosives in at least 11 of the shells on his assembly line. He slipped the note into one of the shells, hoping that someone who benefited from his efforts might discover why.

Elmer Bendiner was the navigator, he told the story of that bombing run over Kassel, Germany

https://fbindependent.com/when-my-daddy-went-to-war-mystery-of-unexploded-shells-p11053-89.htm

https://charlesoheller.com/2014/01/30/czech-saboteurs-in-world-war-ii/

https://www.outono.net/elentir/2023/03/03/the-anonymous-czech-prisoner-who-saved-an-american-b-17-bomber-in-world-war-ii/