Yes, it can happen for multiple reasons. I have personally seen it fail once: a PhD candidate in physics was asked a random but basic question about quantum phy

Yes, it can happen for multiple reasons. I have personally seen it fail once: a PhD candidate in physics was asked a random but basic question about quantum physics, he couldn't answer, and the committee "voted" no. From what I have seen, it is more common for people to simply quit their PhD candidacy once they form their PhD committee, because the committee members will ask for a soul crushing extra amount of work before the defense (happened to two friends of mine).