I have no idea. It's certainly a first for me as well. What's stranger is that I haven't touched it in a year I have no idea. It's certainly a first for me as well. What's stranger is that I haven't touched it in a year+ since replacing the alternator. And I'm fairly certain I didn't over torque it as I likely just snugged it with a small socket wrench. A real head scratcher. I think it may have been all those glorious 1fz horsepowers I was unleashing.
I'm just praying to the cruiser gods that I didn't damage the engine.
If you just snugged it up, then it was likely under-torqued which would lead to a fatigue break. If the washer was already missing, that would also lead to under-torquing.
BTW, bolts don't wear out from being removed & reinstalled to the proper torque. Proper torque only stretches the bolt to 70% of its elastic limit. so it should always return to its original shape.