The diva gave up on her three-song performance in New York's Times Square on DICk Clark's New Year's Rockin' Eve TV special because she couldn't hear her backing tape through faulty in-ear monitors.
After asking fans to sing the tracks, a clearly unhappy Mariah stopped performing altogether and simply grooved awkwardly with her dancers.
She has since accused the producers of the TV special of sabotaging her performance to win ratings, accusations they have firmly denied.
Mel B, who was presenting a rival New Year's Eve show in Times Square, was stunned by what she saw - and didn't hear - and insists Mariah should have done better.
"I was watching, going, 'What is going on over there?' because I was trying to lip-sync along to her song, pretending I was Mariah, and then it all stopped, and then I looked over and I couldn't believe what I was actually seeing," she said during an appearance on U.S. talk show The Real on Wednesday (11Jan17).
"As a performer, as somebody on TV, if something happens, you just keep going. Your first reaction is to not let silence go too long, so I was in complete shock. I was like, 'Why did you let that happen?' And this is Mariah, who's an amazing singer and an amazing performer."
The Wannabe singer then mused, "Maybe she doesn't have that voice anymore. I hate to say it... I think maybe she was having a rough day vocally and didn't even want to attempt that when it actually came to singing live because some of the track was pre-recorded, all of the high notes, and some of it was just blank for her to sing live."
Carey was caught lip-syncing as she stepped out to perform Auld Lang Syne and she had to stop pretending to sing along to her vocal track when she couldn't hear herself through the monitors.
"I want everyone to know that I came to New Year's Eve in New York in great spirits and was looking forward to a celebratory moment with the world," Mariah said in a Twitter audio message on Sunday (08Jan17). "It's a shame that we were put into the hands of a production team with technical issues who chose to capitalise on circumstances beyond our control."