That seemed to spark Watford into life and they got themselves back into the game in the 66th minute when Faraoni headed home an Almen Abdi cross from close range. It looked as though the game was over minutes later when Karacan restored the two-goal advantage with a neat finish on 70 minutes.
But Watford were not done as once again they got themselves back into the game - this time from the penalty spot.
Guthrie pulled Fabbrini down in the area and referee Mike Jones had no hesitation in pointing to the spot.
Deeney stepped up to coolly slot the ball past McCarthy and, with stoppage time fast approaching, Fabbrini prodded the ball past McCarthy from close range to rescue a point.
Reading led three times thanks to a brace from Jem Karacan and an early Adam Le Fondre strike.
But Watford hit back with goals from Marco Davide Faraoni, Troy Deeney and a late goal from Fabbrini to prolong their unbeaten start to the Sky Bet Championship season.
Reading took the lead on eight minutes when Le Fondre fired the home side into the lead in spectacular fashion.
Watford keeper Manuel Almunia came off his line to clear a cross from Jobi McAnuff but could only punch it as far as the edge of the area. .
Le Fondre was waiting to hit a brilliant first-time volley through a crowded penalty area and into the top corner. It was no more than** **Reading deserved as they dominated the early exchanges with McAnuff and Royston Drenthe controlling much of the play for Nigel Adkins' side.