https://arstechnica.com/science/2020/07/independent-reviewers-offer-80-suggestions-to-make-starliner-safer/
Boeing did not run an integrated software test that encompassed the roughly 48-hour period from launch through docking to the station. Instead, Boeing broke the test into chunks. The first chunk ran from launch through the point at which Starliner separated from the second stage of the Atlas V booster.
As a result of this, on the December flight, the spacecraft captured the wrong "mission elapsed time" from its Atlas V launch vehicle. It was supposed to pick up this time during the terminal phase of the countdown, but it grabbed data 11 hours off of the correct time instead. This led to a delayed push to reach orbit and caused the vehicle's thrusters to expend too much fuel. Because of this, Starliner did not dock with the International Space Station.
A second error, caught and fixed just a few hours before the vehicle returned to Earth through the atmosphere, was due to a software mapping error that would have caused thrusters on Starliner's service module to fire in the wrong manner. Starliner was very nearly lost. The combination of these two errors prompted NASA to initiate what it termed a "High Visibility Close Call" investigation.