Watch video footage of the mistake below, as reported by Digital Spy.
Ariana Grande said she was inspired to change her set list for the One Love Manchester benefit concert after the mother of one of the teenage victims said her daughter would have wanted to hear the hits.
The 23-year-old star arrived on stage an hour into the show, for which there were extra security measures in place following Saturday night's terror attack in London.
After all, the performance was being live-streamed across the world so will.i.am could have been addressing London.
The large crowd had fallen silent at the start of the concert to remember those who were killed and injured in both Manchester and London.
Proceeds will go to an emergency fund set up by the city of Manchester and the British Red Cross.
A sell-out crowd of 50,000, many of them clutching "For our angels" signs, fell silent for a minute before Mumford & Sons frontman Marcus Mumford opened the show.
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The group joined up with Ariana to sing a moving rendition of 'Where is the love?' but the front man's apparent monumental cock up, was what really seemed to get people talking.
Thank you. I want to thank you so much for coming together and being so loving and strong and unified.
"Manchester, we're gonna be all right", she yelled. "And unified. I think the kind of love you're displaying is the kind of medicine the world needs now". "The only thing we'll feel here tonight is love, and positivity".
Grande posted on Instagram about the visit, showing her chatting with a young girl in the hospital.
The One Love Manchester concert aired across the globe.
Grande's manager Scooter Braun said on Sunday that the concert will go on as planned.
"I'll be returning to the incredibly courageous city of Manchester to spend time with my fans and to have a benefit concert in honor of and to raise money for the victims and their families", she added.
"We will not quit or operate in fear".
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