Tooth Interior Fatigue Fracture (TIFF) is an emerging gear failure mode with significant implications for gear lifetime and reliability, particularly in modern mechanical transmissions subjected to higher loads, smaller sizes, and increased Noise, Vibration, and Harshness (NVH) requirements. This paper investigates fatigue cracks initiated within the interior of pinion and/or side gear teeth in transmission differentials, representing a new area of recent failure detection. Conventional modeling has suggested that TIFF cracks originate in the case/core area of gear teeth, primarily seen in idler transmission gears. However, this study challenges this notion by presenting evidence that cracks often form closer to the middle of the tooth at various heights. These findings indicate the need for a deeper understanding of TIFF, as the existing theoretical descriptions may not fully explain the observed behaviors. By focusing on this novel area of failure, the paper highlights the importance of addressing TIFF to improve the reliability and durability of gears in modern mechanical transmissions.