Sheared Seatpost

Yeah looks like a small area of fatigue, but no more than 1/6 of the total cross section width. Aluminium's a sneaky customer, you never get something for nothing :-( . Just makes you wonder about all those aircraft flying around with quietly growing fatigue cracks. Or maybe we don't want to think about that...

Believe me, they check aircraft components for fatigue cracks fastidiously.