Simon Mignolet's howler helped West Brom climb into third place in the Barclays Premier League table as Sunderland's hopes of back-to-back victories were torn apart.
The Black Cats keeper gifted Shane Long the simplest of opportunities to fire into an empty net two minutes before the break when he allowed Chris Brunt's pass to squirm from his grasp.
Steve Clarke's men were already leading through Zoltan Gera's superb strike on the half-hour, and although the home side launched a fightback when Craig Gardner's 73rd-minute free-kick was deflected home, substitute Romelu Lukaku's late penalty looked to have sealed a fourth successive win for the first time in the Premier League.
Stephane Sessegnon reduced the deficit from close range with three minutes remaining, but after Lukaku had hit the post deep into injury time, fellow substitute Marc-Antoine Fortune made sure at the death for a 4-2 win.
The Baggies arrived on Wearside brimming with confidence to meet a team which, having secured just its second Premier League win in 19 attempts, ran out determined to start climbing the table amid a run of what they considered to be winnable fixtures.
They did so knowing, however, that they would have to reach the heights they did in the second half of last weekend's 3-1 win at Fulham if they were to emerge with a positive result, and the early signs were positive.
Striker Steven Fletcher glanced a header wide from Phil Bardsley's right-wing cross and Sessegnon, who appeared to have rediscovered his form of last season, forced a less than convincing save from Myhill with a dipping volley after running on to Adam Johnson's flick.
However, high-flying West Brom were making a game of it and midfielder Claudio Yacob had earlier passed up an opportunity to head home Chris Brunt's right-wing corner at the far post, although in his defence, he may not have seen the ball until late after Gareth McAuley had launched himself at it in front of him.
Sessegnon called upon Myhill once again with 25 minutes gone when, despite his lack of inches, he met Sebastian Larsson's free-kick with a downward header.
But any momentum Sunderland had established dissipated on the half-hour when the visitors took the lead in spectacular style.
Johnson could only take the pace off McAuley's long ball out of defence and Gera took it in his stride, turned neatly and blasted a left-foot shot past Mignolet and inside the post.
The Black Cats might have been level within two minutes when the ball fell to John O'Shea inside the penalty area and he curled a right-foot effort just high and wide of the top corner.
Martin O'Neill's men were battling hard for a way back into the game and they went close again two minutes before half-time when Sessegnon's enterprising run ended with a deflected shot which Myhill fielded with little difficulty.
However, their hopes were dealt a major blow within seconds when Simon Mignolet, whose form this season has been a source of comfort for his manager, suffered a calamitous meltdown.
The Belgium international went down to collect Brunt's harmless through-ball as Shane Long close him down, but inexplicably fumbled it into the path of the astonished striker, who was left to tap into an empty net.
O'Shea, who had been booked for an early foul on Long and then held his breath as the striker went down a second time, only to be booked for diving, failed to re-appear after the break as winger James McClean got the nod.
However, the game might have been effectively over within four minutes of the restart when Long ran in on goal, only for Mignolet to partially redeem himself with a vital block.
There was an uncomfortable moment for opposite number Myhill three minutes later when he managed to beat Johnson's long-range effort away after it reared up at him.
But for all Sunderland pressed, the Baggies' organisation and work-rate proved equal to the task, and their ability to counter at pace proved a potent weapon.
They might have extended their lead with 61 minutes gone when Brunt was allowed time and space to drill the ball across the face of goal, where the sliding Long only just failed to make contact.
Peter Odemwingie blasted inches wide after cutting inside from the right with 21 minutes remaining, and that proved to be his final contribution as he and Long made way for Fortune and Romelu Lukaku seconds later.
The Black Cats gave themselves hope when Gardner's 73rd-minute free-kick clipped Fortune in the defensive wall and flew past the helpless Myhill, but Johnson's trip on Liam Ridgewell handed Lukaku an 81st-minute chance from the penalty spot and he took it with aplomb.
Sessegnon pulled a second goal back from close range after Myhill had parried Fletcher's 87th-minute header, but after Lukaku had hit the post, Fortune ended their hopes with a neat finish in the fifth minute of injury time.