https://techcrunch.com/2020/04/22/spacex-engine-issue-on-last-starlink-mission-caused-by-cleaning-fluid-according-to-elon-musk/

SpaceX CEO and founder Elon Musk said on Twitter on Wednesday that the cause of the failure of a single Merlin engine during the most recent Starlink launch (which didn’t prevent the launch from ultimately succeeding at its mission) was the result of an undetected, “small amount” of a learning fluid that ignited during the flight.

The SpaceX Falcon 9 vehicle uses nine Merlin engines on its first-stage, and can still operate successfully in case one stops working. One did stop working during the ascent phase of the Starlink mission that took place on March 18. The engine failure didn’t affect the subsequent deployment of 60 Starlink satellites, which went as planned, but it did prompt an investigation into the cause by SpaceX, which was joined by NASA ahead of the commercial crew flight that will carry NASA astronauts for the first time using a Falcon 9 on May 27.