Stainless steel can have surprising failure modes. For instance, crevice corrosion[1].
The shiny stainless steel fittings holding up your sailboat mast can corrode straight through without more than some tiny telltale signs.
You can see an example here: http://forums.sbo.sailboatowners.com/showthread.php?t=107622 The manufacturing likely provided a tiny crack, splash a little salt water and next thing you know your sailboat's mast is in the water next to it trying to bash a hole in the hull.