A Google self-drive car has been pulled over by police in Mountain View, California, for driving too slowly.
No action was taken but it does raise questions about whether the cars, in their current form, are too cautious.
In a post on Google+, the net giant joked: "Bet humans don't get pulled over for that too often."
An accident report recently filed by the California Department of Motor Vehicles described a Google automated car as "over-cautious".
In a blogpost about this week's incident, the Mountain View police department said an officer "noticed traffic backing up behind a slow-moving car in the eastbound lane".
"The officer stopped the car and made contact with the operators to learn more about how it was choosing speeds along certain roadways and to educate the operators about impeding traffic," it added.
The car was travelling at 24mph in a 35mph zone.
In its own post about the incident, Google said: "We've capped the speed of our prototype vehicles at 25mph for safety reasons. We want them to feel friendly and approachable, rather than zooming scarily through neighbourhood streets."
But it added that, in 1.2 million miles of autonomous driving tests, "we're proud to say we've never been ticketed".
In September, Google said it was working to make its cars drive "more humanistically" following complaints that they were too polite.
Human error Google's fleet of autonomous cars are programmed to follow the rules of the road to the letter but this can cause problems when the vehicles are sharing the road with human drivers who do not.
Researchers in the field have acknowledged that getting autonomous cars to work well in the world of human drivers is one of their biggest challenges.
This problem is illustrated in a recent accident report published by the California DMV which described how a Google AV (autonomous vehicle) and its test driver exhibited "an abundance of caution" at a pedestrian crossing.