Charlie Austin gave Ralph Hasenhuttl his first victory in charge of Southampton and ruined Arsenal's 22-game unbeaten run with an 85th-minute winner in a 3-2 triumph on Sunday.
Danny Ings and Henrikh Mkhitaryan both hit braces before Bernd Leno came and missed Shane Long's cross to allow substitute Austin to head the host to its first victory in 15 outings.
Much of the build-up centred on the role Laurent Koscielny, starting his first top-flight match since April, would play in stabilising a makeshift three-man defence, yet it was he who twice lost Ings for a pair of first-half headers.
Mkhitarayan replied with one of his own before the break and levelled for the second time with a deflected 53rd-minute shot, but the Gunners' second-half fightback stopped short on this occasion as they were left three points adrift of the top four.
Defence comes undone
Arsenal's unfamiliar rearguard first came unstuck in the 20th minute with Matt Targett afforded too much time to measure a delightful left-sided cross to the near post for Ings to break the deadlock.
Hasenhuttl consciously restrained his touchline celebrations and it was clear why as, eight minutes later, Nacho Monreal mirrored Targett's assist, expertly finding Mkhitaryan for a fine nodded finish.
The Austrian was more willing to indulge in delight when Liverpool loanee Ings ghosted between Koscielny and Stephan Lichtsteiner and deftly headed Nathan Redmond's dinked delivery beyond Bernd Leno in the 44th minute.
Alex McCarthy foiled Pierre-Emerick Aubameyang soon after the restart but there was nothing the goalkeeper could do about the second equaliser, Mkhitaryan's effort taking a thick deflection off Jannik Vestergaard.
Shane Long was soon introduced in place of Ings and the fit-again forward provided the cross that fatally exposed Arsenal's aerial vulnerabilities, Austin's winner lifting the struggling Saints out of the bottom three.