Originally Posted by pdlamb

Possible, as in Schrodinger's cat may have slept here. Unlikely. First, no corrosion (rust) is visible, so it wasn't weakened. Second, you mentioned the bag had been mounted on another bike; if it were a case of infant mortality (i.e. the bolt was flawed at manufacture) I'd expect it would have broken previously. Occam's Razor-inspired conclusion is that the bolt unscrewed, probably due to vibrations while riding. That's why I suspect it wasn't screwed in tight enough; that's what torque values on bolts prevent if applied during assembly. And a thread-locker might hold a bolt on that wasn't screwed in tight enough, but it's best to torque the bolt during assembly.