Mark Viduka's late winner for Leeds left Middlesbrough still searching for their first win of the season as the visitors won 3-2 at the Riverside.
It seemed Steve McClaren's men were guaranteed at least a point until Viduka scored in the 89th minute.
Leeds took the lead in the 16th minute when Lamine Sakho, on-loan from French club Marseille, netted.
But Boro hit back with two goals in three second-half minutes from Szilard Nemeth and Brazil forward Juninho.
Another on-loan Frenchman, defender Zoumana Camarra, then levelled for Leeds in the 77th minute before Viduka's goal set the seal on a see-saw match.
Leeds had rejected a loan bid for Viduka from Inter Milan on Friday. And the result meant there was no fairytale scoreline for Boro's former Leeds defender Danny Mills, playing his first game for his new club.
Leeds manager Peter Reid, who could have insisted in a clause in England international Mills's contract forbidding him from playing against his old club, told Sky Sports: "Mills is a Middlesbrough player now. They pay his wages."
Meanwhile McClaren said he was "puzzled" at how his team had lost a match in which they had contributed so much.
"At times we played brilliantly. We had enough chances to win three matches and we lost 3-2. The result is just one of those things that happens in football."
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