Middlesbrough's hopes of automatic promotion faded further at the Riverside Stadium when Brentford came from behind to win 2-1.
Ashley Fletcher's finish in the sixth minute looked to have put Boro on track for a third win in four, but a much-needed victory - due to wins for Norwich, Sheffield United and Leeds - was taken away from Boro in the final 20 minutes, leaving them 12 points off the top two.
Defender Ryan Shotton helped Henrik Dalsgaard's drive into his own net after some poor defending in the 70th minute and that was followed by Said Benrahma's winner three minutes later.
It was only Brentford's second away win of the campaign and left Middlesbrough fans questioning Tony Pulis' decision to replace striker Britt Assombalonga with full-back George Friend when they led.
Fletcher rewarded Pulis' recent faith in him by scoring for a second home game in a row to edge Boro ahead.
The former West Ham striker reacted first in the area to force a finish over the line after goalkeeper Daniel Bentley had spilled Mo Besic's low drive from 20 yards.
That opening goal arrived after Middlesbrough midfielder Jonny Howson and Brentford striker Neal Maupay had seen lobbed efforts drop the wrong side of the upright.
Brentford then had the better of the chances. They had the ball in the net twice but each occasion referee Jeremy Simpson ruled them out.
The first was when Maupay was deemed to be offside as he headed in from eight yards and then the striker was adjudged to have fouled George Saville moments before Sergi Canos powered low into the net.
Canos was the main threat and had two fantastic efforts either side of the break when he must have thought he was going to level things up.
His first was a thunderous drive that crashed against the crossbar and his curling second effort after the interval saw Middlesbrough goalkeeper Darren Randolph turn over with a fantastic one-handed stop.
Boro, whose decision to take off Assombalonga and replace him with the fit-again Friend led to sarcastic chants at Pulis, came within inches of a second when Fletcher headed Shotton's deep cross just wide of the far post.
But Shotton turned into his own net at the other end when Dalsgaard was handed the chance to shoot towards goal after Saville headed weakly away.
Then Brentford took the lead in the 73rd minute. This time Dalsgaard, making the most of freedom down the right, teed up Benrahma in the box to power home first time inside Randolph's bottom corner.
The managers
Tony Pulis: "We are disappointed. They are a good side, three or four players would be in any team in this division. We got a great start, we should have had a definite penalty.
"I will go and see him (the referee), I don't normally. He has booked Jonny Howson for diving and the lad has caught him. I don't know how they get it so wrong sometimes. We should have had three definite penalties. At home you think you might get them, or at least one."
Thomas Frank: "Sometimes stats don't tell the whole story. We knew it was coming and it would be nice to get a win against one of the big teams and this was like a big win.
"It was about time. Boro is a top club and one of the best in England. We knew it would be difficult and we are very pleased with the way we did it. I am proud of the boys, they did very well."