In article ,
says...

With the investigation roughly 80% complete SpaceX believes they know
what caused the explosion during a pad test in April. The current
pressurization design uses titanium check valves between the helium
pressurization system and the propellant tanks. Such valves can allow
flow in both directions, since they can be commanded open before full
pressurization is reached. That is apparently what happened in this
case, where about a cup of oxidizer 'backflowed' onto the fuel
pressurization side and came into contact with titanium valves there,
creating a titanium fire/explosion.

The space people I follow on Twitter are already questioning why a
titanium check valve was used in the first place (posting a link to an
old paper on the fact that NTO can cause a fire with titanium parts if
enough energy is present). I don't know how common titanium check
valves would be in aerospace NTO plumbing (some on Twitter were saying
it's common). I also don't know if this was such a big issue, why
didn't NASA oversight catch this?

Right now, I'm more confused than anything.

To correct the situation, the titanium check valves will be replaced
with 'burst valves'; essentially a pressure plate that breaks when
full helium pressure hits it, which prevents the backflow problem.
Another case of 'simpler is better'.

This sounds like a sane solution, so that NTO will never get into the
helium plumbing by mistake.

Jeff
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