Shambolic defending from Arsenal turned what started off as an open contest at Old Trafford into a remarkable thumping.
Danny Welbeck caught Arsenal’s back-line napping after 21 minutes and headed in Anderson’s pass to put the hosts ahead.
Robin van Persie then squandered a chance to equalize as his penalty was saved by David De Gea before Ashley Young struck to make it 2-0.
Wayne Rooney netted a third for United but Walcott gave Arsenal unlikely hope going into the break with a strike to make it 3-1.
But after both sides had wasted several goal-mouth opportunities, goals from Rooney, Nani and Park Ji-sung resumed the scoring in the second-half before Van Persie made it 6-2 for some consolation.
There was still time for Arsenal to go a man down as usual, as Carl Jenkinson was sent off for a second offense, before Walcott gave away a penalty for Rooney to secure a hat-trick and make it 7-2.
And Ashley Young completed Arsenal’s misery with United’s eighth goal in injury time.
Last time Arsenal ever conceded 8 goals was against Loughborough (8-0) on 12 December 1896, but that was in Division 2.