The guest appearance of all guest appearances took place last night, when Lady Gaga casually popped up on stage, in her 10-heels, to join the lads from U2 in a surprise rendition of ‘Ordinary Love’.
The band were playing at New York's Madison Square Garden when the 29-year-old star joined them to perform the song, which was written in honour of Nelson Mandela for the soundtrack to 'Mandela: Long Walk to Freedom'.
Hugging and kissing the star - who of course sang and played the piano - Bono said: "It's yourself. Her Ladyness, Her Gaganess is here this evening. That's a treat."
At the end of the performance, he added: "Princess, Queen, one day Woman of New York City, here with us tonight in Madison Square Garden. What a delight."
And after the performance, the 'With or Without You' singers took to Instagram to praise the singer once again.
Along with a photo of them on stage, they wrote: "All hail Her Gaganess @ladygaga#U2ieTour."
Replying to the message on Twitter, Gaga wrote: "@U2 all hail You Too."
@U2 all hail You Too— Lady Gaga (@ladygaga) July 27, 2015
The 'Do What U Want' singer isn't the first famous face to have been spotted at one of the band's recent concerts, with Sir Paul McCartney, Nile Rodgers, Alicia Keys and Jon Bon Jovi all attending the group's shows.