Watford stunned FA Cup holders Arsenal with a 2-1 quarter-final victory that piled further pressure on Arsene Wenger at a mutinous Emirates Stadium.
Recent slip-ups in the Premier League title race and the likelihood that Barcelona will complete a two-legged Champions League victory over Arsenal this week left a third consecutive FA Cup triumph as the most realistic prospect of silverware for long-serving boss Wenger this season.
But they failed to make first-half dominance count, with Mohamed Elneny the main culprit in terms of missed chances, and Watford - without a goal in their previous three matches - capitalised on growing anxiety after the break.
Hornets top scorer Odion Ighalo ended a run of 599 minutes without a goal with an opportunist strike before Adlene Guedioura hit a spectacular second after 63 minutes.
Substitute Danny Welbeck reduced the arrears with two minutes to play and then struck the post in a breathless finale, but Wenger's men could not avoid a first FA Cup defeat since Blackburn Rovers left north London with a shock win in February 2013.
Arsenal striker Olivier Giroud had the ball in the net after nine minutes but he was marginally offside when collecting Alexis Sanchez's throughball.
Nathan Ake made a vital challenge in the Watford area with Joel Campbell poised to open the scoring, although the hosts were struggling to make much of an impression in the Watford area,
Home centre-back Gabriel was incredibly fortunate his rash, two-footed lunge on Troy Deeney went unpunished, while Elneny's woeful 30th-minute hack over the Watford bar lacked such danger but was similarly unaccomplished.
Elneny endured another shocking miss following fine build-up play from Mesut Ozil and Campbell, and goalkeeper Costel Pantilimon's yellow card for time-wasting at the subsequent goal-kick underlined Watford's desire for the interval to arrive.
Giroud headed over from a left-wing corner as Arsenal resumed on the front foot but Ighalo stunned Emirates Stadium in the 50th minute.
Deeney flicked on Ake's long throw at the near post and Ighalo rolled Gabriel to steer a shot beyond David Ospina on the turn.
The Watford strikers combined to similar effect two minutes later, with Gabriel again coming off second best as Ighalo shot over.
Arsenal were rattled and Ighalo showed some of the form behind his recent drought when he misjudged a lunging attempt in the 60th minute - the impressive Ake having raided town the left.
That wastefulness was almost punished as Pantilimon made a stunning reaction save to deny Giroud at his near post, but Watford grabbed the breathing space they required.
Once more, Deeney was the provider - his excellent hold-up play crowned by a measured lay-off for the onrushing Guedioura to clatter an unstoppable drive into the roof of the net from the right corner of the area.
Etienne Capoue prevented Giroud from converting a Calum Chambers cross at the near post and the France striker made way as part of triple substitution that saw Theo Walcott, Danny Welbeck and Alex Iwobi introduced.
Welbeck stroked a measured finish into the far corner when he collected Ozil's clever backheel and the former Manchester United man should have spared the Wenger some awkward questions when he crashed over a minute later - his initial drive having cannoned back off the post.