OK, so combining Charlie's comment about the apparent ease of inadvertently clicking off an IPhone photo with the fact there is a mirror on the mantel, my next question is did you approach the mantel/mirror immediately before or after taking the mantel photo?
Images in mirrors when seen (photographed?) from oblique angles can be distorted, the degree of distortion came be a function of several variables including distance, age, lighting, materials, craftsmanship etc. For this distortion to offer a possible explanation for the mystery image, however, would have required you to have had the camera just inches from the mirror at a pronounced oblique angle relative to the plane of the mirror. If you did not have the phone close to, but at an angle from, the mirror, this theory is unlikely.