The Destiny's Child bandmates hit Sing Sing karaoke bar in South Beach on Wednesday night and rented a room for an impromptu jam session.
But when they overheard girls in the other room belting out "Party," one of Bey's hits, they had to get involved.
"Beyonce's room was facing another room where three random girls were singing a Beyonce song," a waiter tells Celebuzz. "So they just walked in and said 'Hey girls!'"
At first, the ladies didn't even realize it was Beyonce and Kelly joining in their singing. "One girl was like, 'That looks like Kelly Rowland,'" the waiter says, before they recognized they were actually singing with the artist being played.
The excited fans got photos with the stars after their surprise duet together. One girl was passed out on the bench and B and Kel even photobombed her!
Best night ever, or what?! We're jealous!
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Gallery | Beyonce's Red Carpet Style