I've seen it happen VERY early in Rebellion though.
The last game I played, the final AI surrendered with 14 capitals, a lvl8 titan, and several star-bases still left. Now could I have killed him, no doubt (max fleet with 14 lvl10 caps + lvl10 titan). But if he'd parked all of his capital ships, his (loyalist none-the-less) titan, and the rest of his fleet all on top of a star-base (or two even being loyalist faction) he surely could have made it a hell of a final stand against even my armada.
I hadn't even sent a single ship into one of his gravity wells yet when he surrendered. That seems somewhat pre-mature with what he had remaining. (And no way to know what I had, I was still far enough away he couldn't see my ships jumping)