The 'X Factor' 2010 band were using a New York gym when Liam Payne came face-to-face with an admirer while he was stripping down.
Apparently she managed to slip past bodyguards as they were busy holding back an eager crowd in the city.
A source told The Sun: "Liam was stunned when he saw her in the changing room - he couldn't believe she'd got past security.
"For one thing, he was in the middle of getting dressed and had no idea what she was going to do, as their fans in the US are mad."
However security was soon alerted and the female was escorted out by minders.
Liam and bandmates Harry Styles, Niall Horan, Zayn Malik and Louis Tomlinson were also allegedly put on the spot on Wednesday night by Will Smith's son.
Jaden Smith spotted them at the 'Men In Black 3' premiere party and took them to his actor dad to rap the 'Fresh Prince Of Bel Air' theme tune to him.
Reports suggest he'd seen their cover of it online and helped them carry out the performance but the film star embarrassingly forgot Zayn's name.