Lando Norris admitted he was "pretty annoyed with myself" for the track limits transgression on his final lap that cost him a career-best third in Emilia Romagna GP qualifying.
Playing a starring role for McLaren throughout Saturday's action at Imola, Norris briefly put himself into a provisional second place on the front row just behind Lewis Hamilton with a stunning lap of 1:14.454 at the end of Q3.
But the time was swiftly deleted by race control after Norris ran too wide over the white line denoting the edge of the track out of Piratella.
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Had the lap stood, Norris then would have ended up splitting the Red Bulls on the grid in third place. Instead, with his original Q3 time standing, he will start behind team-mate Daniel Ricciardo in seventh place.
Norris' could not hide his frustration after, telling Sky F1 he was "pretty disappointed, pretty annoyed with myself".
"It was a very good day until then," he said.
"The team did an awesome job. The car really came alive in qualy, we made a lot of improvements in Friday, and that one lap that I don't need to make a mistake on I effed it all up."
He added: "It's the same for everyone. In the end of the day, it's my mistake and I've messed it up. From the outside it looks a lot easier to just stay within the lines than what it is inside [the car].
"When you are in qualy and you are in Q3 and you have an opportunity to be P2, P3 or score your best result in qualifying, you're pushing. When you're going at such speed and such a high-speed corner, misjudging it by this [narrows fingers] amount..."
The Englishman, who finished a fine fourth at the season-opening Bahrain GP three weeks ago, has only previously started in the grid's top three owing to penalties for others.