A rotational panoramic radiograph with an apparently severe error in patient positioning was obtained. There was no image in the middle part of the film which was black. The origin of this severely degraded image is explained by simulating the position of the jaws in relation to the path of the beam during the exposure. The error was caused by incorrect positioning of the patient so that the X-ray beam actually passed outside the jaws when the anterior region was radiographed. Similar misalignment could be reproduced using a phantom head which had been displaced 3 cm lateral and 1 cm behind of the ideal position.