My grinder seized up mid-bloom on a Sunday, and a $4 part saved $200

Last Sunday I was pouring a V60 for my wife and me when my hand grinder just locked up halfway through 20 grams of beans. I was in the kitchen at 7am, still half asleep, and I thought the burrs were done for good. Turned out a single chunk of a rock, about the size of a peppercorn, had wedged itself in there. I pulled the burr set apart, dug it out with a paper clip, and it spun free again, no damage. Then I weighed my beans first instead of eyeballing them, and I found three more little stones in that same bag. I now dump every new bag onto a white plate before grinding, and I've caught four rocks in six bags from that same roaster.