The accident occurred in a testing site for space equipment. The high pressure combustion took place in a liquid hydrogen tank (H2) under a high pressure and in presence of 12 m (700 kg) of liquid H2. The test was stopped as the malfunction was detected by the sensors, without consequences. The cryogenic tank, with a nominal working pressure of 250 bar was connected to H2 gas and liquid oxygen storage tanks. Few months before, the test tank was emptied and purified (by injecting helium) as the H2 gas network was suspected of being polluted due to an intake of air from a compressor, one year before.The day of the accident, a test was performed: the liquid H2 tank was pressurized by a control device (up to 250 bar in 30 sec). When the pressure in the tank reached 160 bar, a noise indicating a pressure drop was observed and the detectors recorded a pressure fluctuation of 10 bar. The sudden and significant fluctuations of the measured parameters (temperature of wall, pressure, flow) brought to the termination of the test. The cause of the accident was the presence of combustion of frozen oxygen in an blind branch of the tanks drain pipe. Its presence was probably due to the partial unsuccessful purging by helium, which had not been able to purge the air contained in the blind pipe, because placed in the lower part of the tank. The trapped air had re-solidified when while refilling the tank with liquid H2. Several combustion spots were detected: in condensed phase between solid O2 and H2 gas, in less dense phase between H2 and O2 gas. The combustion was undoubtedly triggered by the friction of the pellets against the walls and the adiabatic compression of the H2/O2 gas mixture.