A rare meeting between two self-driving cars resulted in one takingevasive action, Reuters reports.
The robot cars, one made by Delphi Automotive and one by Google, met on a Californian road in Palo Alto.
The Google car pulled in front of the Delphi vehicle making it abandon a planned lane change.
The incident comes as Google's purpose-built self-driving cars take to California highways to see how well they mix with regular traffic.
Emergency encounterDetails of the encounter were revealed to Reuters by John Absmeier - director of Delphi's autonomous car driving unit.
The vehicles involved were conventional road cars modified with lasers, radar, cameras and other sensors to help them navigate roads without a driver.
The incident occurred as the Delphi car, an Audi Q5 crossover, was preparing to change lanes. As it did so the Google car, a Lexus RX400h crossover, abruptly moved in front of it forcing the Audi to abandon its manoeuvre.
The Delphi car coped well with the incident, said Mr Absmeier, and "took appropriate action". A Delphi spokeswoman clarified the incident with tech news site Ars Technica saying there was nothing amiss in the encounter and that its vehicle behaved "admirably".
Google played down the the incident, saying early reports that the cars were involved in a "near miss" were inaccurate. It said the cars treated each other as they would any other vehicle and neither was in danger of colliding with the other.
Delphi and Google's autonomous vehicles have been involved in several minor accidents and incidents during testing. However, before now all of those have involved the robot cars and human-driven vehicles. In almost all cases, the firms have said, the fault lay with human drivers.
The encounter occurred earlier this week shortly before Google's purpose-built robot cars began to be tested on roads in Mountain View close to the search giant's headquarters.
Google has created a webpage through which people can share their encounters with the cars and their impressions of how they drive.
A monthly report about Google's self-driving car project reveals that it is currently testing 32 autonomous vehicles - 23 modified Lexus SUVs and 9 purpose-built prototypes. During testing, it said, these vehicles have covered more than 1 million miles (1.6 million kilometres).
It also detailed some of the accidents in which the cars have been involved. Many of these resulted in minor damage to the Google cars and the most serious involved a driver in a modified Lexus taking control to avoid a human-driven car that ignored a stop sign.
The report also reveals how the cars coped when they met other road users such as emergency vehicles and cyclists at night.