Cradling his head in his hands, Sam Allardyce couldn't believe Yann M'Vila's inexplicable decision to commit to an utterly needless tackle.
M'Vila's foul - which presented Southampton with their match-winning penalty - left Sunderland heading into the two-week international break four points adrift of safety after a third defeat in four in Allardyce's reign.
It was difficult to argue that Southampton were the better and more proficient side in a dire spectacle at the Stadium of Light, with Sunderland mustering just one meaningful effort on goal.
Sunderland were indebted to a couple of smart saves from Costel Pantilimon, plus a pair of superb goal-line clearances from DeAndre Yedlin and Sebastian Coates for getting to the midway point of the second half with the scoreline still goalless.
But yet another self-inflicted blow from Sunderland scuppered Allardyce's game-plan of keeping proceedings tight and attempting to nick a winner, as Dusan Tadic swept home from the penalty spot.
Sunderland - who handed a first Premier League start to Duncan Watmore - had a big let-off in the 15th minute when Coates was caught sleeping to Tadic's header and Graziano Pelle got to the loose ball first on the left-hand side of the six-yard box.
Pelle flicked it elegantly into the path of Tadic, but his low shot was excellently blocked by Pantilimon.
The Romanian had to produce another smart save midway through the half when Sunderland's failure to deal with a left-wing cross saw the ball bounce invitingly for Sadio Mane 15 yards out.
Mane connected with a powerful half-volley, but Pantilimon managed to beat the ball away.
But the first half, as a whole, was a desperate spectacle, with Sunderland happy to keep a compact defensive shape, while Southampton enjoyed the lion's share of possession.
Allardyce made a change at the interval, with Fabio Borini replacing the ineffective Ola Toivonen - a switch which saw Adam Johnson moved into the 'number 10' role.
Yedlin showed enormous courage to prevent the deadlock being broken 10 minutes into the second half when he headed Jose Fonte's flick-on off-the-line, as Mane tried to pounce.
Sunderland again cleared off-the-line on the hour mark when Tadic's left wing cross was only headed clear as far as Steven Davis 12 yards out, at the far post.
Davis' shot had beaten Pantillimon, but Coates did magnificently to read the effort and boot clear from a yard out.
But Southampton took the lead from the penalty spot in the 69th minute after M'Vila committed an utterly needless foul on the left-hand side of the box when Ryan Bertrand was going nowhere for the visitors.
Tadic coolly converted the penalty, placing it in the right-hand corner, where Pantilimon couldn't reach it, despite guessing the right way.
Sunderland immediately responded with their first meaningful effort - the ball bouncing into the path of Borini to the right of goal, whose well-hit half-volley was turned away by Maarten Stekelenburg at the near post.
Johnson's 25-yard free-kick then just cleared the crossbar by a couple of inches, with Stekelenburg stood motionless.
Sunderland: Pantilimon, Yedlin, Coates, Kaboul, Jones, M'Vila, Gomez (Defoe 74), Toivonen (Borini 46), Johnson, Watmore, Fletcher (Rodwell 70). Subs not used: Mannone, Brown, van Aanholt, Lens. Booked: Johnson (43)
Southampton: Stekelenburg, Yoshida, Clasie (Romeu 78), Fonte, S Davis, Mane (Jimenez 83), Tadic (Caulker 90), Ward-Prowse, van Dijk, Pelle, Bertrand. Subs not used: K Davis, Cedric, Martina, Seager. Booked: Mane (11), Bertrand (74), Pelle (90)