Sory Kaba's stunning overhead strike sealed a vital win for Cardiff City in their Championship survival fight.
Cardiff came from behind at Vicarage Road after Ismaila Sarr tapped in his 11th of the season for an early Watford lead.
Kion Etete and Cedric Kipre both scored in the space of three minutes before Kaba's acrobatics before half-time.
The result saw Cardiff climb two places clear of the relegation zone and put a major dent in Watford's play-off hopes.
That both teams are on their third manager of the season speaks volumes for the instability of these clubs.
Sabri Lamouchi will be encouraged that Cardiff can see out their final four games to avoid the drop with rivals QPR and Reading both dropping points.
But while Chris Wilder's side may be 14 points further up the table, the manner of this capitulation will do little to appease the Hornets faithful.
Not that many saw it coming after half an hour.
Victory over Bristol City on Saturday had not only ended a run of eight games with a solitary win but also relieved the pressure on Wilder.
It had also even roused play-off hopes and they looked full of confidence as they strolled through a static Cardiff defence for the opener on 11 minutes.
Kaba was wasteful in midfield and Imran Louza had time to float his pass to the far post where Keinan Davis headed back across goal and Sarr tapped in.
The goal saw Cardiff briefly slip into the Championship relegation zone and they would have been two goals down if not for Kipre's last-ditch tackle on Hassane Kamara.
But while Watford threatened in attack, there was a vulnerability at the back though few could have predicted the implosion before half-time.
Against the run of play, Etete showed delightful footwork to cut inside and shoot past Daniel Bachmann for his first goal since December.
Moments later and Ryan Andrews' foul handed Cardiff a free-kick. Ryan Wintle's blocked effort fell kindly for Kipre to smash home a second.
Watford were reeling but could only stand and watch in awe when Kaba controlled Perry Ng's cross on his chest and - with his back to goal - scored with an outrageous bicycle kick.
In fairness, the travelling Bluebirds supporters were just as surprised given Cardiff had been level with bottom side Wigan Athletic as the division's lowest scorers before kick-off.
Cardiff signed Kaba on loan from Danish side FC Midtjylland on the final day of the January transfer window and has now scored seven goals in 10 starts.
The visitors could - or probably should - have grabbed a fourth when Mark McGuiness shot narrowly wide from a corner and the ire of the home fans was clear as the boos rang out for half-time.
Wilder brought on Ken Sema at half-time but the Sweden international horribly scuffed Watford's best chance of the second half after Joao Pedro's clever pass.
Sarr shot tamely straight at Ryan Allsop late on to sum up a spiritless performance.
Watford: Bachmann, Andrews (Ngakia 65'), Porteous, Hoedt, Kamara (Asprilla 65'), Choudhury, Louza, Koné (Sema 46'), Sarr, Pedro, Davis.
Subs not used: Hamer, Kabasele, Araújo, Bacuna.
Cardiff City: Allsop, Romeo, Ng, Kipré, McGuinness, O'Dowda, Wintle, Ralls (C), Rinomhota (Sawyers 68'), Etete (Wickham 68'), Kaba (Harris 82').
Subs not used: Alnwick, Sang, Ojo, Davies.