Arsenal's defence crumbled for the second time in a week to hand Swansea victory at the Liberty Stadium.
The Gunners conceded three late goals to gift Anderlecht a point in the Champions League on Tuesday night and Arsene Wenger's men capitulated in the Barclays Premier League on Sunday as Swansea came from behind to claim a 2-1 victory.
Alexis Sanchez had put Arsenal ahead with his 12th goal of the season, but Gylfi Sigurdsson scored a curling free-kick and substitute Bafetimbi Gomis headed home Jefferson Montero's cross with his first touch to give the Swans the win.
The result means Swansea leapfrog Arsenal into fifth position, level on points with fourth-place West Ham.
Newcastle are now level on points with Manchester United (who Jose Mourinho says are still in the title hunt) and only one point behind the Gunners.
It was also an afternoon to forget for Arsenal's local rivals Tottenham, who lost 2-1 at home to Stoke.
Bojan Krkic scored his first goal since joining Stoke from Barcelona and Jon Walters added a second before Nacer Chadli pulled one back for Spurs, who are now just five points outside the relegation zone in 12th.
Elsewhere, Everton denied Sunderland a second successive win thanks to Leighton Baines' 76th-minute penalty.
Baines fired past Costel Pantilimon from the spot to make it 1-1 after Connor Wickham had fouled Seamus Coleman in the area.
Sebastian Larsson, with a free-kick, had put the Black Cats ahead earlier in the second half with his first goal since the opening day of the season.