Aberdeen were punished for "a few moments of madness", according to manager Derek McInness, following a 2-1 defeat by Motherwell.
The Dons slipped four points behind Premiership leaders Celtic after Motherwell came from behind with goals from Scott McDonald and Louis Moult.
Defensive lapses led to both goals for the hosts at Fir Park before Aberdeen lost Barry Robson to a red card.
"We beat ourselves today," McInnes told BBC Scotland.
"We know the importance of getting that second goal and I thought we looked likely to get it.
"Motherwell put you under pressure and you never think you are home and dry at 1-0."
'Too much deliberating'
A Kenny McLean penalty separated the sides at the interval, with the visitors well on top in the first 45 minutes.
But Motherwell improved after the break to make it five wins in six matches, although they were given a helping hand by some poor defensive work from the Dons.
"There was too much deliberating and hesitancy when we lose the first goal. We should deal with the ball bouncing about," said McInnes.
"The second goal deserved to be punished. We were in control of the situation and we've made it easy for Moult to lift it over.
"Then we get Robson sent off and all of a sudden we go from doing well and managing the game to more of a challenge."
Robson was ordered off moments after coming on as a substitute for aiming an elbow at McDonald, McInnes saying of the experienced midfielder: "He should know better."
It was just a second defeat in 18 games for the Dons, who have played a game more than Celtic.
On Aberdeen's title challenge, McInnes added: "There wasn't a lot of room for error before today.
"We've got 21 points to play for and I still think there will be twists and turns for us.
"We feel we can contest every one of those games and we'll see where we finish."
The opening goal upset Motherwell boss Mark McGhee because he thought the officials missed a foul for his team in the build-up.
"We were a bit aggrieved about the nature of the penalty because we felt we should have had a free-kick," he explained.
"But Aberdeen were at least one goal better than us in the first half and I thought we lacked a bit of belief.
"Second half, we pressed them more and it was a terrific performance. We showed great spirit and scored two good goals.
"Our shape was better and we managed to get Scott McDonald and Louis Moult up against their centre-halves."
Motherwell's excellent run has taken them up to fifth place but McGhee remains wary of the teams below in a congested mid-table.
"We can start to think about the top six if we win another game but we have a difficult trip to Inverness then it's Celtic here," he added.