Simone Inzaghi bemoaned Inter's 2-1 loss to fierce rivals Milan and admitted his players learned a "tough lesson" as the Scudetto title race was blown wide open.
Inter were heading seven points clear at the top of Serie A with a game in hand after Ivan Perisic's first-half volley put them in control of Saturday's Derby della Madonnina.
The Nerazzurri continued to dominate for large parts of the match and would have been further ahead if not for Mike Maignan, who pulled off four big saves in the first half.
However, Inter were punished for failing to kill off the contest when Olivier Giroud scored a quickfire double at San Siro in his first taste of this famous fixture.
Giroud slid in at the back post to level up the game on 75 minutes and added a second three minutes later, becoming the first Frenchman to score twice against Inter in Serie A.
After falling to a barely believable first home league loss since October 2020 – also against Milan – Inzaghi was left to wonder just how his side chucked away the points.
"This is football. We dominated in every way but weren't sharp at the end. They were lucky on the first goal and were smart on the second," he said.
"I saw a derby with one-way traffic for 60 minutes. In the final 25 minutes, we weren't as sharp or as in control, and the game was more open to individual incidents.
"We know it's a defeat that hurts, but strong teams must be able to analyse their errors.
"If we played it again 10 times, we might lose it once. We all know how important the derby is; we didn't deserve to lose, but that is football.
"Until the equaliser, we had allowed Milan practically nothing. It's a tough lesson, but a valuable one to make us more determined to kill off games so this doesn't happen again."
Inter's players were unhappy that Giroud's first goal was allowed to stand as they felt that he barged into former Arsenal team-mate Alexis Sanchez earlier in the attack.
But Inzaghi accepted Inter only had themselves to blame after falling to just a second league loss to Milan – who lost Theo Hernandez to a late red card – in their last 12 meetings.
"The lads got irritated at the equaliser, as there was probably a clear foul on Alexis Sanchez and we lost our focus," he said.
"We should've scored the second goal, while Alexis Sanchez was barged off the pitch practically by Giroud on the goal.
"But the fact remains if we had taken more of our chances, [the Giroud incident] wouldn’t have mattered. It's not a problem, it was a challenge.
"If you think it's fair, that's fine. It's not the issue – the lads lost their focus a bit after that incident and that's the problem. The referee let a lot go."
Saturday's defeat was Inter's first to Milan in the league after leading in the game since February 2004 and leaves them just one point clear, albeit with a game in hand.
Samir Handanovic, whose 30 Serie A appearances against Milan is the most of any current player against another in the competition, echoed the thoughts of his head coach.
"Today we did everything. We had the game under control and played well," he told Inter TV. "But then we lost a few too many balls and were punished.
"There are many more games and the mistakes have to be reviewed. It must be understood why we managed the last 15 minutes like we did.
"Sometimes you take risks and you are not punished; today we took risks and were punished."