A clip posted online from their Cardiff gig shows the star forgetting the words to the Wheatus song, which the group are performing on their current world tour.
He said: "Okay, I've just forgotten the lyrics. I'm not even going to pretend I know them!"
One Direction's tour reaches Dublin's O2 Arena tonight (March 6) before heading to Belfast, Manchester and Sheffield and returning to London on April 4.
It was recently reported that the band are hiring military security for their shows after a shoe was thrown at Harry Styles during a US gig.
Photo gallery - One Direction on the 'Take Me Home' tour: