Last night SpaceX conducted a Cryo Pressure Test on the fuel tanks of its version three prototype Starship. The previous version, SN2, was destroyed in a Cryo Pressure Test a couple of months ago. The goal of these tests is to achieve sustained cryogenic fuel pressures of 8.5 bar, the benchmark NASA uses for human space flight certification. The pressure required for this design for launch and flight is 6.5 bar but of course they have to meet the safety standard of 8.5 bar. The previous SN2 ship attained the pressure required but blew out where the engines would normally be attached, an area called the thrust puck. In this latest design, SN3, there was a new thrust puck area, and large hydraulic rams to simulate engine thrust pressure on the lower assembly. That lower area seems to have survived this last test, but the upper tank bulkhead or a valve in that area failed. Elon is claiming that there was a test procedure mistake, but they will need to do a post mortem on the remains before anything concrete can be fixed. You can see the failure here:
https://youtu.be/wFXQ5SRCy74
Note that the crumbling of the structure is caused by the sudden pressure wave generated when the liquid nitrogen is suddenly released at the top, there is no explosion.
It is not unusual for prototype designs to be tested to the point of destruction to provide design feedback, and demonstrate absolute limits. It could be that SN3 failed past the certification pressure point, but we don't have those details yet. In any case I bet Elon is pissed, as it throws his schedule for a 20km demonstration flight, out the window. And achieving orbital flight of this prototype this year is looking doubtful.