Slow leaks. They usually aren't covered by insurance and many can be asymptomatic. That is what precisely happened to one of our clients. The plastic water supply line to their refrigerator suffered a pin-hole leak, causing water to slowly drip under the laminate flooring under the fridge and then eventually into the kitchen space.
Our client noticed nothing until months later when the boards started to buckle and there was a slight darkening present in the wood. Once our inspector tested the floor with a moisture meter, we trace the flow of the water back to the fridge and eventually discovered the leak.