Manchester - Jose Mourinho accused his Manchester United players of shooting themselves in the foot as he saw his under-strength team gifted Arsenal two goals in a 2-2 draw that extended their winless Premier League run to four games.
United, who started with Paul Pogba and Romelu Lukaku on the bench, showed great spirit to twice come back, with equalisers from Anthony Martial and Jesse Lingard earning a much-needed point.
But, after a defeat at Manchester City and dispiriting draws with Crystal Palace and Southampton, United are now 18 points behind leaders City and actually closer to the relegation positions than they are first place.
United also trail Chelsea and Arsenal, who are now unbeaten in 20 games, by eight points in the battle for a place in the top four.
And Mourinho, who saw a bad David de Gea error and Marcos Rojo own goal twice hand Arsenal the lead, believes the performance continued their run of self-inflicted harm.
"We scored four goals and we drew 2-2," joked Mourinho.
"But we scored four goals and even in matches like today where we play well, we always shoot ourselves.
"Sometimes we miss chances with an open goal, other times defensive mistakes but tonight there was a fantastic spirit, good performances.
"Our problem is finding a performance like this without the mistakes we made and we had very good performances where we make mistakes and we are always punished."
Mourinho's frustration was especially understandable given that Arsenal's opening goal, after 26 minutes, featured that rarest of collector's items, a mistake by his Spanish international goalkeeper David de Gea.
Chris Smalling slipped at Lucas Torreira's 26th minute corner, allowing Mustafi the space to send a free header bouncing into the ground and goal wards.
It looked a routine save for United's number one but he succeeded only in pushing the ball into the air, two-handed, over his own head and over the goal line, despite Ander Herrera's desperation clearance.
But United responded within four minutes as Rojo, playing for the first time this season, unleashed a terrific free-kick which Bernd Leno kept out at the foot of his left-hand post.
Herrera responded quickly, although replays suggested he may have been fractionally offside, reaching the loose ball on the by-line and turning it into the six-yard area where Martial was able to convert.
That dramatic pattern would be repeated after 68 minutes when Arsenal regained a lead they could only hold, on this occasion, for 74 seconds.
Lacazette played a one-two with fellow substitute Henrikh Mkhitaryan and advanced into the area where Rojo tackled him from behind and succeeded only in turning the ball into his own net.
But there could be no faulting United's response as Arsenal panicked at a long ball punted forward, almost directly from kick-off.
Lukaku caused uncertainty in the visitors' area and as Sead Kolasinac tried to turn the ball back to his goalkeeper, he allowed Lingard to ghost in and complete a simple finish past Leno.
Error-strewn as it might have been, there could be no faulting the entertainment in the first Manchester United-Arsenal fixture since 1986 to take place without either Sir Alex Ferguson or Arsene Wenger in one of the dug-outs.
De Gea made amends for his first-half error with two fine saves to deny in-form Pierre-Emerick Aubameyang, with Mkhitaryan also wasting a good close-range chance in between.
Neither Mkhitaryan nor the injured Alexis Sanchez have shone since a January swap deal between the clubs, but only the offside flag denied the Armenian coming back to haunt United in stoppage time when he rifled home after another decent De Gea stop from Torreira.
"I am proud of my players and the second half gives us confidence," said Arsenal manager Unai Emery.
"We controlled the game and if one team deserved to win, we did more."