Anybody who owns and uses a shot bag for bending metal has done this. You're busy hammering away and suddenly the bag's material splits or gets punctured, spilling out all that heavy lead shot. You have to stop the metal work and gather up the messy innards, spend time repairing the bag and then get back to work.
We learned years ago that as heavy-duty as shot bags are, they can't take the pounding very long without tearing. That's why we always put a scrap piece of leather or heavy vinyl between the bag and the metal piece we're working with. It cushions the bag and, as an added bonus, dampens the noise.
Don't bother covering the entire bag with leather or vinyl. That will just give you a false sense of security and you'll eventually tear through the protective material. Just lay your piece on the bag and keep it handy when the bag is not in use.