Arsenal manager Arsene Wenger slammed his side's defending after watching Anderlecht claw their way back from three goals down to earn a 3-3 draw in Champions League Group D.
The hosts were 3-0 ahead inside the hour and apparently cruising into the last-16 of the competition for a 15th successive season under the Frenchman when the wheels came off in spectacular fashion at The Emirates.
Instead of shutting up shop, Wenger's side capitulated in worrying fashion, allowing the Belgian champions to hit back with two goals from Anthony Vanden Borre, the second a penalty conceded by Nacho Monreal, before Aleksandar Mitrovic's 90th minute header left the home crowd stunned.
Elsewhere, holders Real Madrid breezed into the last-16 with two games to spare when a Karim Benzema strike secured a 1-0 win in their Group B match at home to a toothless Liverpool.
Benzema clipped a Marcelo cross high into the net from close range in the 27th minute, the France forward's fifth goal in this season's ompetition, to maintain Real's perfect record in the section with 12 points from four games.
It was the record 10-times European champions' 12th win in a row in all competitions and followed their 3-0 success against Liverpool at Anfield last month.
FC Basel are well placed to join Real in the knockout round after they beat Ludogorets 4-0 to move onto six points, with Liverpool and the Bulgarian champions on three each.
Juventus 3-2 Olympiakos
Malmo 0-2 Atletico Madrid
Borussia Dortmund 4-1 Galatasaray
Benfica 1-0 Monaco
Zenit St Petersburg 1-2 Bayer Leverkusen