Explosion on the Saturn stage

[Note HIAD validator: this an incident where the hydrogen systems were involved as effects of the initiating event. There is nevertheless and additional possible contribution by hydrogen embrittlement]
During preparations for static firing tests of liquid oxygen - liquid hydrogen S-IVB stage, an explosion occurred prior to the lift off command. All systems indicated normal values. Extensive damage was received by the test stand and stage. Sides of the stage and liquid hydrogen tankage were fragmented. Many small fires around stand. Estimated fire ball diameter was about 300 feet. Propellant mixing resulted in a detonation estimated to be 1 percent of the TNT equivalent.

Explosion caused by failure of high pressure titanium spheres used to store gaseous helium. The tanks, located below the thrust ring, failed due to the use of incorrect and out of specification filler wire in the fabrication. Results of incorrect wire were to lower the ultimate strength and introduce hydrogen embrittlement. Subsequent explosions were considered effects rather than causes.

LH2 tank, rocket stage