QPR's remarkable victory leaves Watford without a home win since 20 October.
Martin Rowlands' penalty gave QPR an early lead, with Damion Stewart heading home a corner for the visitors' second.
Rowlands burst into the area and fired in a third on 40 minutes, before Watford's Damien Francis finished from close range early in the second half.
QPR substitute Buzsaky stole in and rounded Richard Lee before angling home a drive to make it 4-1, with Danny Shittu poking in Watford's second goal.
Watford manager Adrian Boothroyd:
"We dominated for long periods in the first half and yet ended up 3-0 down at half-time.
"I was pleased with our attacking play but not some of the defending. It was schoolboy stuff.
"There was no ranting or raving at half-time, we just talked about what we had to do to. We got back into it and then left ourselves open."
QPR manager Luigi De Canio:
"I think the team is playing better and better. We took our chances which meant the losses of concentration we sometimes have didn't cost us.
"In the dressing room we only talk about our opponents, not what might happen tomorrow or in January.
"Everyone needs to do his duty. That is a sign of maturity and each one of these players has that."
Watford: Lee, Doyley (O'Toole 79), DeMerit, Jackson (Shittu 46), Stewart, Smith, Williamson, Francis, McAnuff, Darius Henderson, Ellington (King 68).
Subs Not Used: Mariappa, Ainsworth.
Booked: Doyley, Smith.
Goals: Francis 52, Shittu 84.
QPR: Camp, Barker, Rehman, Stewart, Malcolm, Bolder, Rowlands, Ainsworth (Vine 64), Balanta (Buzsaky 73), Nygaard (Walton 90), Blackstock.
Subs Not Used: Cole, Moore.
Booked: Nygaard, Barker.
Goals: Rowlands 13 pen, Stewart 29, Rowlands 40, Buzsaky 80.
Ref: Chris Foy (Merseyside).