Lots of people have considered the carbon fiber igniting in a helium
bottle rupture.
http://spaceflightnow.com/2016/09/23/falcon-9-rocket-explosion-traced-to-upper-stage-helium-system/
Not that you need the carbon to catch fire to ruin your day. The
bottle popping is more than enough.
I rather imagine SpaceX is busy testing their 5500 psi helium bottles
to see what temperature cycling does to rupture strength.
But there is another consideration, the supercooled LOX. Supercooled
LOX is going to cause oxygen from the atmosphere to condense and run
down the tank. Water and CO2 will freeze, nitrogen will not condense
so whatever runs off is going to be mostly oxygen.
I presume they thought this through before they went with supercooled
LOX, but you never can be sure..
Keith
