Brass will get harder and brittle the more times it is used. Always inspect your brass before and after resizing. Thats where polishing comes in handy. Makes cracks and flaws easier to see. A crack can cause a case split or seperation, but more than likely won't blow up your gun. If you have cases that are cracked or you think they may be cracked, don't shoot them. Brass is too cheap to take chances with.