Strange. Their previous failure (CRS-7 blowing up in flight in 2015.[1]) was supposedly caused by a liquid helium tank support strut in the second stage breaking in flight. The liquid helium tank is inside the liquid oxygen tank, so a failure there overpressurizes the liquid oxygen tank.[1]

This second failure again involves a similar liquid helium tank. Not one under flight stress, just sitting there on the pad. Similar failure, though - tank leaks, overpressurizes liquid oxygen tank, second stage explodes.

The Saturn V had helium tanks inside the liquid oxygen tanks, but they were compressed gaseous helium, not liquid helium. Has any other rocket design ever put a liquid helium tank inside a liquid oxygen tank?

[1] https://www.nasaspaceflight.com/2015/07/spacex-falcon-9-fail...