Sialia sialis (Eastern Bluebird) and Passer domesticus (House Sparrow) are common cavity-nesting birds that can compete with each other for nest sites in habitats where they overlap. Both species occasionally lay eggs in the nests of conspecifics, but evidence of interspecific brood parasitism (IBP) is lacking. Here we report the first detailed case of an Eastern Bluebird laying an egg in a House Sparrow nest and discuss potential explanations, including intentional IBP, IBP by default, and failed nest usurpation.