Happened to me before.

But I luckily noticed right away. But its not really something that Google could completely prevent.

Yes, but that's true of anything, right? You could accidentally misread a traffic signal and drive into the middle of oncoming traffic. But like with sharing on social news sites, you can usually recover quickly enough that it's not a big deal. Slam the brakes, delete the post, etc.

People make mistakes and no software can prevent that.

Account != circles, if you are using the wrong account public sharing does not have the same meaning anymore.