As you probably know, the WWII slides were only spot-hardened. With extensive firing, they would crack right where they weren't hardened. You could also just about watch these slides 'peen' as you fired the gun. The post-war slides were much better, but still a bit soft for use on a match pistol, for instance. The USGI National Match slides were quite hard, and I can't recall ever seeing a cracked one.