There was even a case of eleven twenty-millimeter cannon shells penetrating the fuel tanks of a B-17 flown by the 379th Bomb Group without exploding. Inside one of the shells, taken by technical intelligence for examination, was a note in Czech that read 'This is all we can do for you now.' This was in Winged Victory, talking about the high amount of duds that the Germans were using mainly due to slave labor. What are the odds? That's a pretty neat little story.