Kate had appeared her usual elegant self as she arrived at the charity event for Shout, which provides a 24/7 text support service for anyone in crisis.
The couple were in good spirits, and the Duke of Cambridge was even seen reaching for his wife's back and rubbing it lightly in a rare PDA as they entered the Troubadour White Theatre to meet with volunteers working with the organisation.
But it was when they were leaving the venue that the Duchess's misstep occurred.
Having said goodbye to organisers, Kate, who was carrying a bouquet of beautiful flowers, was ushered towards one door of the waiting royal car, as William went around to get in the other side.
Kate then accidentally tripped, as captured in the video (above) almost falling over.
William snapped into action and appeared to reach his hand out to his wife, however she quickly regained her balance and immediately laughed at her stumble.
The couple then shared a quick smile before ducking into the backseat of the car.
During the event, Prince William allegedly expressed interest in becoming a crisis counsellor.
After meeting at the Shout event she said Prince William is "trying to figure out how to work the training into his schedule. But he’s quite sincere about it and is certainly knowledgable."