Abdoulaye Doucoure's controversial last-gasp equaliser denied Southampton a first win in 10 matches.
The midfielder diverted the ball in with his arm a minute before full-time as Watford came from two goals down to snatch a 2-2 draw.
James Ward-Prowse had scored twice in the first half before Andre Gray pulled one back - and Saints were crestfallen when Doucoure's equaliser was allowed to stand.
He stooped to nod into the net - but the ball missed his head and he swept it home with his arm.
Southampton protested long and hard to referee Roger East but to no avail, with a draw leaving them just one point above the relegation zone.
Watford 2-2 Southampton
