Size M5 x 0.8 mm, class 8.8 metric screws were failing during application, reportedly at the normal installation torque. Investigation (visual inspection, metallographic analysis, and unetched 8.9x fractographs) supported the conclusion that the fasteners failed via ductile overload in the absence of gross defects or embrittlement. It was subsequently determined that a nonapproved lubricant had been used during installation. Tension preloads can be more than twice their normal level on lubricated fasteners because of reduced friction, and in this case, the preload was sufficient to fracture the screws. No recommendations were made.