“Crimp jump,” also known as “bullet creep” occurs when a bullet creeps forward out of the casing too far (protruding past the cylinder face). This causes it to hit the back of your barrel when you cock the revolver, thus tying up the gun and rendering it inoperable. “Crimp jump” is caused by loading a heavy bullet into a semi-lightweight revolver, therefore leading to extra recoil.
User error can sometimes contribute to “crimp jump.” If reloading a revolver before all rounds have been shot off, remember to rotate the cylinder so that the older rounds fire first. Exposing older rounds to more recoil will eventually lead to “crimp jump.”