Brentford scored two goals in the final 10 minutes to come from behind and deny Watford victory at the Community Stadium.
Bryan Mbeumo slotted in a stoppage-time penalty to make it 2-1, minutes after captain Pontus Jansson had equalised with a header at the back post.
Watford led for an hour thanks to Emmanuel Dennis' first-half header but a late surge from Brentford was eventually rewarded.
Defeat means Watford remain just three points above the relegation zone.
Dennis, who has been directly involved in nine of Watford's last 12 Premier League goals, was left unmarked as he nodded in the opener against the run of play in the first half.