Simon Mignolet's howler helped West Brom climb into third place in the Barclays Premier League table as they beat Sunderland 4-2.
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 the win. 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.
The Baggies made a positive start but 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.
But any momentum the Black Cats had established dissipated on the half-hour when the visitors took the lead in spectacular style.
Adam Johnson could only take the pace off Gareth McAuley's long ball out of defence and Zoltan Gera took it in his stride, turned neatly and blasted a left-foot shot past Mignolet and inside the post.
Sunderland's hopes were dealt a major blow when 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 Chris 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.
The Black Cats gave themselves hope when Craig Gardner's 73rd-minute free-kick clipped sub Marc-Antoine Fortune in the defensive wall and flew past the helpless Myhill, but Johnson's trip on Liam Ridgewell handed Romelu Lukaku an 81st-minute chance from the penalty spot and he took it with aplomb.
Stephane Sessegnon pulled a second goal back from close range after Myhill had parried Steven 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.