Certify's unbeaten record went up in smoke as she came home a disappointing fourth behind L'Amour De Ma Vie in the District One-sponsored Balanchine at Meydan.
The winner of each of her four starts as a juvenile, the Godolphin-owned filly missed her entire three-year-old campaign following a lengthy suspension due to the Mahmood Al Zarooni drugs scandal.
She made an impressive start for trainer Charlie Appleby with a dominant display in the Cape Verdi three weeks ago and was a 2/5 favourite to make it six from six in this nine-furlong Group Two.
The warning signs were there for her supporters before the off as Certify appeared reluctant to enter the stalls before eventually being loaded in a blindfold.
She travelled well in the hands of Mikael Barzalona for much of the race, but was unable to quicken in the straight and was ultimately well held.
L'Amour De Ma Vie, trained in France by Pia Brandt, finished second behind Certify in the Cape Verdi, but comprehensively reversed the form, scoring by a length and three-quarters in the hands of Maxime Guyon.
Fellow French challenger Flotilla was second, building on her Cape Verdi fourth, with Eddie Lynam's Irish runner and Cape Verdi third Pearl Of Africa occupying the same position again.
Guyon said: "She ran very well last time. She finished close to Certify and tonight she won easily.
"She's improving all the time and it's my first winner in Dubai, so I'm very happy."
There was some consolation for Godolphin as Tasaday claimed the Listed Dubai Millennium Stakes in the hands of Silvestre De Sousa.
The winning rider told Sky Sports: "She's done it well and has come on a lot from her last run.
"I think she would appreciate some give in the ground, but she's got the class so she got away with it."
Mick Halford and David Marnane have both enjoyed excellent Carnivals so far and between them supplied another couple of winners for Ireland.
Halford's Eastern Rules (5/1) ran out a decisive winner of the District One Handicap in the hands of Shane Foley.
The six-year-old finished third on his Dubai bow in January but struck gold earlier this month on the Tapeta and followed up in good style with a length-and-a-quarter verdict over Sommerabend on turf.
Foley said: "It was a big step up today and there were a couple of class horses in the race, but he showed he's improving.
"It was his first run on grass over here, but I always thought he might be a bit better on grass. Thankfully it's worked out.
"We've had a handful of runners (at the Carnival) and if they haven't been winning, they've been placed.
"I'm not complaining and it makes my job a lot easier."
Elleval won at Meydan little over a year ago and made it third time lucky at this year's Carnival in the District One Phase I Handicap.
The 16/1 shot dug deep for Fergal Lynch to get the better of Start Right by a neck.
Marnane said: "He loves the weather here, like all of us.
"He has a great turn of foot and I think he's better on Tapeta so we will sit down and have a think about (future plans).
"We will see what the handicapper does and go from there."
Medicean Man (9/2) was another winner at the 2013 Carnival who got his name on the scoresheet this year with a neck verdict over Ahtoug in the Meydan Sobha Phase II Handicap (Turf) under Harry Bentley.
Winning handler Jeremy Gask said: "He's a dream to train with great owners. He's just one of those horses that comes along for a trainer and you wish you had a couple more of them.
"He didn't get the cover we perhaps wanted, but he stuck his neck out and it's wonderful to be here.
"Where we'll go from here, we'll see."
Avon Pearl (16/1) created a little bit of history as he became the first Norwegian-trained Carnival winner in the Meydan Sobha Handicap.
Partnered by Pat Dobbs, the five-year-old enjoyed a nice run through and battled on well to fend off Van Rooney by two and a quarter lengths.
Trainer Rune Haugen said: "It's a fantastic thing for us to be here and win.
"We had some good runs at Ascot and won the (Norwegian) Derby, but this is huge."
Dobbs added: "I was drawn well and he jumped well. I got a bit further back than I wanted, but the gaps came and he went through them very bravely."