That’s an especially serious problem because there’s another question that Biden knew was coming Thursday, though it was mostly posed through passive-aggressive comments: Is he simply too old and off his game to be the Democratic presidential nominee? Rambling rants about turntables certainly don’t help put such concerns to rest.

But Biden didn’t stop there. Perhaps he sensed how poorly the answer had gone and was flailing, but he then decided to answer several foreign-policy questions that had been asked of his rivals earlier in the night.

“No, I’m going to go like the rest of them do, twice over,” he said when the moderators tried to stop him—the flash of anger a rare break in his unfailing politesse on the debate stage.

“By the way, in Venezuela, we should be allowing people to come here from Venezuela. I know [President Nicolás] Maduro. I’ve confronted Maduro. Number two, you talk about the need to do something in Latin America. I’m the guy that came up with $740 million, to see to it those three countries, in fact, changed their system so people don’t have a chance to leave. You’re all acting like we just discovered this yesterday.”

Now Biden wasn’t just rambling—he was the angry old man demanding his rivals, and the moderators, get off his lawn. It was a disastrous answer. Nor was that the only time on Thursday when Biden seemed to be spinning his wheels as he searched for an answer.

Biden faced other tough jabs from his opponents throughout the night. Most pointedly, Julián Castro accused Biden of forgetting a statement about health care. “Are you forgetting what you said two minutes ago?” Castro said, not very subtly broaching the age question. “Are you forgetting already what you said just two minutes ago? I can’t believe that you said two minutes ago that they had to buy in and now you’re saying they don’t have to buy in. You’re forgetting that.”

(As New York’s Adam K. Raymond pointed out, though, it was Castro who was mistaken—and Biden who’d been consistent.)

Will any of this matter? It’s anyone’s guess, but the evidence so far suggests it might not. There’s an active conversation about whether Biden simply isn’t up to the job, especially in the press, but so far his polling has remained remarkably durable. While there’s heavy churn among his rivals, with the field starting to sort out into some clear tiers, Biden has consistently led by a wide margin. Yet he also cannot put the questions to rest, and with every episode like his record scratch giving more ammunition to doubters, the risk is that the accumulation of missteps starts to drag his numbers down.

If there is one silver lining for Biden, it’s that the misstep came near the end of the debate. Just a few minutes later, he was the first candidate to answer a question about overcoming adversity, and he talked about the death of his wife and daughter in a car crash just after he was elected senator, and the death of his son Beau Biden from brain cancer in 2015. These are tales that Biden has told before, and he tells them well and movingly.