Over the years I've seen a small number of seatposts cam out of frames, but they've all been cases of very shallow insertion depth, usually as little as one diameter or less.
Much more common failures in my experience were posts that were fitted blindly following the marked minimum insertion line, but not deep enough with regard to the frame design. It's become very common to extend seat tubes well above the top tube joint creating a new type of possible failure. Some frame designs reinforce the extended section, but most don't, so a post can be inserted 2-1/2 inches and still end at or above the highest weld.
With all the possible variations so seat tube design, it's more important than ever that end users understand the basics and think about how it applies to their frame.
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