|"Is this all real? Or has this been happening inside my head?"
The topic of this article is of a real-life subject that has been mentioned "in-universe" in a canon source. The Harry Potter Wiki is written from the perspective that all information presented in canon is true (e.g., Hogwarts really exists), and, as such, details contained in this article may differ from real world facts.
In 2003, Clapham Common was a location chosen to place a Portkey leading to a Celestina Warbeck concert. It thus became notable as one of the few sites where Portkeys have been taken by Muggles, as a pair of Muggle dog-walkers found themselves accidentally transported to the concert when their dogs ran off with the old trainer being used as the Portkey. Their presence was not detected for some time, and one of the Muggles even performed a duet of "A Cauldron Full of Hot, Strong Love" with Warbeck. Though the Muggles were sent back to Clapham Common with their memories removed, the Muggle who had preformed the duet would later write a popular song bearing great similarity to Warbeck's.