Harry was singing "What Makes You Beautiful" when Liam took it upon himself to show Hazza's adoring fans even more of the boy bander.
As you can see in the video, Liam yanked down Harry's pants on stage, revealing his boxers!
We're wondering how Liam got Harry's signature tight pants down though--that's no easy feat, we're sure.
The show must go on, however...and Harry continued to sing while pulling his pants back up.
Liam better watch his back though--we're pretty sure Harry's plotting his sweet revenge!