A group of driverless Cruise robotaxis blocked traffic in San Francisco for hours on Tuesday evening after the cars stopped working without explanation.

Details of the incident were shared on Reddit (and spotted by TechCrunch) and illustrate how driverless vehicles are still experiencing teething problems in real-life settings.

Cruise, which is backed by General Motors and Honda, has been testing its technology in San Francisco since February, but only launched a commercial robotaxi service last week. The cars have no human safety driver at all but operate under certain restrictions. They only offer lifts on “select streets” between the hours of 10PM and 6AM when the weather conditions are favorable, and can drive no faster than 30mph.

Pictures of the driverless roadblock show at least five vehicles stopped in the street. The redditor who shared the pictures said the incident took place around midnight.

“The first thing I say to my coworker is that they’re getting together to murder us,” said redditor seansinha in a comment. “It was a pretty surreal event. Humans had to come and manually take the cars away. Cruise should get fined to shit for blocking the street off for so long. They even made it so the street sweeper couldn’t hit an entire block.”

Seansinha said Cruise employees showed up to the incident “in twenty minutes” but that it took them a long time to actually move the cars. “It was a huge debacle,” he said.

A spokesperson for Cruise acknowledged the incident in a statement given to TechCrunch, but offered no details on why it happened, and if the problem could occur again in the future.

“We had an issue earlier this week that caused some of our vehicles to cluster together,” said the spokesperson. “While it was resolved and no passengers were impacted, we apologize to anyone who was inconvenienced.”

It’s not the first mishap involving Cruise’s driverless tech. Earlier this year, a police officer tried to pull over a Cruise robotaxi, apparently because the car’s headlights were turned off at night. Cruise later said the incident was due to “human error.” “Our AV yielded to the police vehicle, then pulled over to the nearest safe location for the traffic stop, as intended,” said the company. “An officer contacted Cruise personnel and no citation was issued.”