Justin Bieber, Katy Perry, Coldplay, Robbie Williams, Pharrell Williams, Miley Cyrus, Take That, Usher, the Black Eyed Peas, Little Mix and One Direction's Niall Horan were all expected to hit the stage.
Watch video footage of the mistake below, as reported by Digital Spy.
The 23-year-old Grande visited injured fans in the hospital last week before Sunday's One Love Manchester concert.
Nearly 15 years after this song was originally released, the lyrics felt more relevant than ever last night.
After all, the performance was being live-streamed across the world so will.i.am could have been addressing London.
Those who were at Grande's May 22 concert when the suicide bombing took place were offered free tickets; the remainder went on sale for $52.
The show will be broadcast across the globe and proceeds will go to an emergency fund set up by the city of Manchester and the British Red Cross.
Thousands pack streets of Huddersfield to celebrate promotion If you look at the players we have, I think they have done very well to finish third in the league and then to reach this final. So, now its time to rest and get ready for the 2017-2018 season, time to improve, reflect and get where we want to be.
The show opened with a minute of silence for those killed and injured in the attack.
In one heartfelt moment, Grande was joined on stage by children from a local school, some of whom were at the targeted Manchester Arena concert, as the group performed her hit song My Everything.
The singer appeared to get emotional as he told those gathered that "God loves you so, so much". "I love you guys so much and I think that the kind of love and unity you're displaying is the kind of medicine that the world really needs right now".
Ariana and her manager Scooter Braun began planning the One Love gig just days after the attack on her Manchester gig, and her fellow musicians were full of praise for how she handled the tragedy.
Grande told the audience that her song selection was influenced by the mother of 15-year-old Olivia Campbell, who died in last month's bombing.
"We're here to show our support to Manchester more than anything".
"My heart, prayers and deepest condolences are with the victims of the Manchester Attack and their loved ones".
Fans seemed to have forgotten any lingering fears about security towards the end of the evening, with a lively crowd of middle-aged fans singing "Tonight, I'm a rock and roll star" as they made their way out of the stadium.
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