This was a game of two halves - after dominating the first half we let the Belgians back into the game to grab a point. This was a team that had never won a game in the UK.
We looked assured and raced to a 3-0 lead, but we were tactically naive and as usual our frail defence let us down. Why couldn't we consolidate our lead at 3-0, sit back, keep possession, and let Anderlecht chase us around the park? - it's not the Arsenal way, but on this occasion it was what was needed. We can't play in this style without the defensive qualities to back it up.
We'll get murdered when we eventually face a quality side - no doubt about it.
Report:
The Gunners scored twice in the last three minutes to win 2-1 in the reverse fixture a fortnight ago and Anderlecht exacted sweet revenge with an incredible second-half comeback at the Emirates Stadium.
Mikel Arteta's penalty and Alexis Sanchez's brilliant volley put the hosts firmly in the driving seat at half-time before Alex Oxlade-Chamberlain looked to have sealed all three points with a third early in the second half.
Anthony Vanden Borre gave the visitors hope just after the hour with a close-range header and the former Portsmouth defender then added another from the penalty spot to make it 3-2.
Anderlecht poured forward in the last 10 minutes and finally got the equaliser their courage deserved as substitute Aleksandar Mitrovic headed home in the 90th minute.
Arsenal remain second and in a strong position to qualify from Group D but their chances of going through as pool winners are all but over as they now trail Dortmund by five points with two games to play.
That will be a particular blow to Arsene Wenger's side, who would have been eager to top the group having qualified second in three of the past four seasons before losing in the last 16 on each occasion.
Arsenal host Dortmund next before an awkward final group fixture away to Galatasaray.
Arsenal made an assertive start but for all the hosts' possession it was Anderlecht who went closest in the opening 10 minutes.
Welbeck carelessly gave the ball away and former Arsenal trainee Dennis Praet almost made the striker pay with a stinging shot that deflected off Per Mertesacker and edged just past the near post.
Anderlecht continued to look dangerous on the break and almost took the lead again when Sacha Kljestan met Cyriac's scooped cross but his close-range shot was dealt with by an excellent Wojciech Szczesny save.
With 20 minutes played, Arsenal carved out a clear chance as Calum Chambers darted down the right and pulled back for Sanchez, who fooled his opponents with a superb dummy before fizzing an angled shot off the post.
Arsenal began to establish a foothold and in the 24th minute they finally made the breakthrough as Mbemba was caught the wrong side of Welbeck in the area and bundled the striker to the floor.
The referee pointed to the spot and Arteta coolly chipped the ball down the middle to score his first goal of the season and give the home side the lead.
Anderlecht had enjoyed a promising opening half an hour but five minutes later they were two behind after a storming Sanchez run earned a free-kick on the edge of box.
The Chilean took the kick and while his initial effort struck the wall, he instantly volleyed the rebound hard and low past Silvio Proto for his fifth goal in three games.
Praet had a chance to pull one back on the break but he took too long to get his shot away while Welbeck almost made it 3-0 at the other end but also took a touch too many and Mbemba was able to make a last-ditch tackle.
The second half began much as the first had ended as Arsenal poured into the Anderlecht box, and as the ball spilled out to Kieran Gibbs he generously teed up Oxlade-Chamberlain whose shot was blocked.
Shortly before the hour-mark, Oxlade-Chamberlain did find the net as the excellent Sanchez hustled in midfield, winning the ball for his team-mate to drive down the left and apply an emphatic finish into the far corner.
Just as the match looked to be all but over, Anderlecht instantly pulled a goal back as Vanden Borre converted substitute Andy Kawaya's cross to give the visitors hope.
The Belgians then reduced the deficit to one goal as Nacho Monreal wrestled Mitrovic to the ground and another penalty was awarded, which Vanden Borre calmly converted to set the home fans' nerves jangling.
Full of confidence, Anderlecht went in search of a dramatic equaliser as Frank Acheampong sent a sizzling long-range shot just over in the 80th minute.
Arsenal had surrendered control of the game and Mitrovic ensured the incredible comeback was complete as he headed past Szczesny in the 90th minute.
Lukas Podolski had a chance to snatch victory at the death for the Gunners but he pulled his shot wide and Arsenal had to be content with a point that leaves their hopes of winning the group hanging by a thread.