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.

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'.


--
"Insisting on perfect safety is for people who don't have the balls to
live in the real world."
-- Mary Shafer, NASA Dryden