In the middle of April, Honda Motor CEO Takahiro Hachigo hopped into a self-driving car prototype made by General Motors’ Cruise Automation for a demonstration ride. It didn’t go well. About 20 minutes in, the car’s software suddenly turned itself off even as the car kept moving. A man sitting behind the wheel—the backup driver—had to take control. Attempts to restart the system failed, and a second Cruise vehicle had to pick up Mr. Hachigo to finish the demonstration.

The previously unreported glitch was embarrassing for GM, Cruise’s majority owner, as Honda is an investor in the company, which was valued at $19 billion in its latest fundraising. More significantly, though, the software outage and previously undisclosed internal data highlight the technological challenges faced by Cruise that have forced it to repeatedly delay the planned launch of a fully automated robotaxi service to the public, from the original time frame of 2018 to the very end of 2019, according to people with knowledge of the matter. Cruise’s difficulties are just the latest reminder of how far self-driving car technology still has to go before being ready for broad commercial use.