I'm like Kenny and use the coarsest grit on a disc sander, and take it down pretty quick, and have never had that. Like Robert says, it looks like the glass is fractured. Was it ever twisted or maybe bent funny in a box or on a shelf? I've seen spots like that in clear glass before. The couple I had was on the end and I cut them off. But it went about halfway through the thickness of the glass and looked like a crack or fracture.