Most hens have brains the size of peas... don't depend on them to know a good egg from a bad one. They'll kick out both from time to time. I've had them kick out or crush open perfectly good eggs and sit on bad ones for 28 days. Some hens will scoot out eggs that have gotten too chilled for whatever reason. Sometimes that's a bad egg, sometimes it got too far from mom. I candle to see if an egg left aside has developed, sit it in the incubator to heat it for 4-6 hours and then check for movement. If movement then I incubate it from there or put it back. Some hens are almost always right but most are just luck of the draw, peabrained, egg sitters, who can and do make mistakes.