Discovered during rollout:
The issue is in the plane’s flight-control computer software. It was confined to how it performs validation checks during startup and doesn’t involve its function during flight, the people said. The problem came to light when the latest version of the software was loaded onto an actual aircraft, according to one of the people. While it has been tested on planes in flight, most of the software reviews have occurred in a special simulator used by engineers on the ground.
https://www.seattletimes.com/busines...eings-737-max/
How was that: After all that scruntity, most extensively tested and safest piece of software ever!