I've seen it before on a heavy use routed iron man and on the magnet on dirty harry. The core mushroomed over the wood and the kept mushrooming down into the wood compressing the wood fiber. Once it's done it's to late even filling with clear can chip easily because of how thick the clear is. Have to live with it or get a new playfeild. This is why magnet core manitance is so important. I remove and regrind mine once or twice a year and my games don't get much play.