Joe Biden appeared to freeze for several seconds before briefly stumbling on his words at an event marking the anniversary of Juneteenth.
The president, 81, stood motionless while people cheered around him at a concert on the White House lawn on Monday.
When he later gave a speech about Juneteenth – for which there is a federal holiday next week – he stumbled through a few words that are difficult to understand on audio.
It comes after Donald Trump, 77, suffered his own mishap at a campaign event on Sunday.
A teleprompter malfunction in Las Vegas sent the former president off on a bizarre rant about his dislike of sharks.
In an attack on clean energy, Trump discussed his concern that electric boats’ weight could make them prone to sinking.
“What would happen if the boat sank from its weight and you’re in the boat and you have this tremendously powerful battery, and the battery is now underwater, and there’s a shark that’s approximately 10 yards over there?” Trump said, according to US media.
“By the way, lot of shark attacks lately. Did you notice that?”
“I’ll take electrocution every single time,” Trump added, in reference to the battery in his hypothetical sinking ship scenario. “I’m not getting near the shark.”
Donald Trump speaks at a rally in Las Vegas. Pic: AP
Why does America mark Juneteenth?
Juneteenth is also known as “Juneteenth Independence Day”, “Freedom Day” or “Emancipation Day” and is celebrated on 19 June each year.
Abraham Lincoln’s Emancipation Proclamation did not free slaves overnight. After it was issued on 1 January 1863, it took over two-and-a-half years for it to become a reality.
Throughout that period, thousands of black Americans did not know about their freedom. Enslaved labour continued in southern states because of resistance to the proclamation.
Juneteenth celebrates 19 June 1865, when federal troops arrived in Galveston, Texas, bringing freedom to the last remaining slaves.
“Black history is American history,” President Biden told the crowd on Monday.
He warned that some “old ghosts” in new clothes – a veiled reference to some of his Republican rivals – seek to take away their freedoms by making it harder for black Americans to vote.