I guess it failed the test..................
Sunday’s explosion, which was first reported by The Washington Post, occurred before the engine was lit, a source told Ars. It happened during a procedure known as LOX drop, in which liquid oxygen is added to the engine to determine if there are any leaks. At that point, something caused the fluids within the rocket engine to ignite. Testing of the Block 5 Merlin engine will be suspended until the cause of that ignition is found and fixed.
https://arstechnica.com/science/2017/11/an-experimental-spacex-rocket-engine-has-exploded-in-texas/
Leak test involving LOX flow to a prototype 5 series engine. Oxygen reacted with something in the engine, as no fuel was involved in the test. Contaminant?