On 20 March 2020 SpaceX successfully launched a batch of satellites, but an anomaly caused one of the 9 rocket engines in the booster to shutdown during flight and the booster subsequently failed to land. A recent tweet gives a cause as being the presence of solvent in a sensor line.
SpaceX drive launch costs down by landing and reusing boosters - the first stage. The booster uses all 9 engines in its primary role, then 3 to decelerate and to land. Only 3 of the engines have the equipment to relight. Many flights have now successfully landed the booster, many boosters have been reflown and some have been reflown several times. In this case the engine shutdown was one of the 3, which precluded successful deceleration at re-entry.
More on the technology and history:
More on this launch:
via this discussion: