As in, had a go of catching Harry in bed. Not a go on Harry in bed. Although you never know.

“We’ve all seen each other’s willies," he told We Love Pop magazine.
"I possibly caught Harry with a girl once. I had an idea that something might be going on, so I thought it’d be banter to go in and ruin it.
"All I did was walk I and shout “Hello!” and then walk out. I’m such a kn*b.”

Aww Louis, you're not a kn*b - well, maybe sometimes you are, but you're a lovely kn*b.