That's happened to me a few times. I fixed mine by sticking the wire back in place and sewing up the hole where it poked through. If the cloth is really worn, it can be patched up with a little piece of scrap material.
It also depends on the ends of the underwire - some of them have a little plastic cap, which can wear/fall off, and makes it pretty much impossible to stop the metal cutting any new fabric you patch over.
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