Automotive and tech executives have long promised that autonomous vehicles would drive better than humans, but that wasn’t the case for a fleet of Cruise cars in San Francisco this week.

The company, a majority-owned subsidiary of General Motors, confirmed it had an “issue earlier this week that caused some of our vehicles to cluster together.” A company spokesman said the problem was “resolved and no passengers were impacted,” but declined to give further details.

Photos and a description of the Cruise robotaxis blocking several lanes of traffic in San Francisco were shared on Reddit and Twitter. At least seven Cruise vehicles can be seen clustering in the intersection of Gough and Fulton streets in the city’s Civic Center neighborhood late Tuesday night, potentially blocking traffic both ways on one of the streets.

It’s unclear how long the cars were blocking traffic or what caused the situation with the vehicles that now provide a service to paying customers on some San Francisco streets.

The incident is another example of how difficult it is to develop and deploy self-driving vehicle fleets. Commercializing autonomous vehicles has been far more challenging than many predicted even a few years ago. The challenges have led to a consolidation in the autonomous vehicle sector after years of enthusiasm touting the technology as the next multitrillion-dollar market for transportation companies.

The San Francisco Department of Emergency Management was notified and contacted Cruise about the problem, San Francisco Police Department public information officer Kathryn Winters told CNBC. She said no officers were dispatched to the scene.

The incident occurred roughly a week after Cruise became the first company to offer unmanned fared rides to the public in a major city. The vehicles operate between 10 p.m. and 6 a.m. on designated streets in San Francisco. 

Alphabet-backed Waymo has reportedly had similar problems involving the clustering of its autonomous vehicles in the city. KPIX-TV, a CBS affiliate in San Francisco, reported in October that Waymo vehicles were getting stuck down a dead end street.

Cruise’s problem also comes months after an online video revealed a Cruise autonomous vehicle getting pulled over by police. In the video posted on April 1, the Cruise car initially pulled over to the side of the road and stopped as the officer approached the driver’s side before it sped across an intersection and went further down the road.