But not to worry, It happens to all of us sometimes. The older I get, the more it seems I spell like that too! 
When I was in the artillery (late '60's), all the shells we fired had to rotate a set times before they were armed and I'd expect it was standard in all countries forces. I don't know about WWII ammo but even if the Germans had such a tech first, it's such an obvious safety factor that I can't imagine the allies would have been far behind in developing their own 'arming mech.' once they knew about it.
My father flew Pathfinder Lancasters during the war and he says the bombs had to rotate before arming (the tail fins made them spin like an arrow) so for sure the allies had the same concept.
However, the gun is still jammed even if the warhead hasn't armed.