Until this month, Google could boast that its self-driving cars had logged more than a million miles without causing a single accident. Google's cars had been involved in a few accidents, but all of them had been the fault of the other driver — often a car rear-ending the Google car.

But on Valentine's Day, Google suffered the first accident where one of its self-driving cars was at least partially to blame.

Here's what happened, according to Google's official report to California regulators. The Google car was driving in the rightmost lane as it approached an intersection, but it encountered a sandbag blocking its path and had to stop. The car tried to merge into the slow-moving traffic on its left to get around the sandbag. It tried to merge in front of a bus, expecting the bus to slow and let it in. But the bus kept moving, and the Google car hit the side of the bus, damaging "the left front fender, the left front wheel, and one of the driver's side sensors."

Obviously this isn't great news for Google, which doubtless enjoyed being able to boast that its cars had never caused an accident. But as accidents go, this one seems pretty minor.

Google estimates that the bus was moving 15 miles per hour at the time of the crash, while the Google car was traveling 2 miles per hour. No one was injured.

And if Google's description is accurate, it sounds like the kind of mistake humans often make: misjudging the likely reaction of another human driver.

On the other hand, the public isn't likely to accept self-driving cars that are merely as good as their human competitors. To convince customers to let computers drive them around, Google is going to have to design cars that drive much better than a human being would — and we just don't have enough data yet to be sure whether self-driving cars are safer overall. So the Mountain View company still has some work to do.

(Hat tip to Kevin Roose.)

Disclosure: My brother is an executive at Google.