Defensive errors cost Brentford dear as Bury came to west London and left a freezing Griffin Park with all three points.
Two goals in 90 seconds from Nicky Adams and Glynn Hurst gave the visitors a platform and when Andy Bishop added a third late in the first half The Bees were presented with a very stiff challenge.
Brentford battle gamely, and got a foothold in the game after a half time shake-up, but couldn't climb the mountain and when Bishop added a late goal the game was up.
Bury had already caused problems for The Bees defensive line, Hurst forcing Ben Hamer into an early save, before Adams put them ahead, capitalising on a mix-up at the back to lash home from 14 yards.
Alan Connell had fired over early on and Wayne Brown had brought a save out of Darren Randolph before the goal and Glenn Poole forced another immediately after the opening strike but Bury soon had their second.
Bishop was the supplier, as he was for Adams' goal, picking up a long pass and slipping it to Hurst, who slotted the ball past Hamer and into the bottom corner.
Brentford tried to fight back but it was the visitors creating the chances, Adams shot weakly at Hamer, Bishop headed wide and Elliott Bennett failed to test Hamer with a low effort and eight minutes before the break Bury grabbed a crucial third goal.
The home side played themselves into defensive trouble and when Hamer tried to release the pressure he only played the ball to Bishop who lofted the ball back over the goalkeeper's head and into the net.
Connell and Robert Milsom had efforts before the break but the half time introduction of Ricky Shakes and Reuben Reid improved The Bees no end.
Poole volleyed narrowly wide and Connell had a shot blocked before Brentford found a way back into the game.
Nathan Elder had an effort cleared off the line when Reid nodded down a Gary Smith corner and when the ball was put back in Efe Sodje was penalised for handball, Poole sent Randolph the wrong way from the spot.
The Bees, now playing three at the back, were dominating possession and Randolph had to produce a brilliant save from Reid to maintain Bury's two goal advantage.
But the visitors were breaking well and Hamer had to smother at the feet of Paul Scott when the midfield man looked set to add a fourth.
Connell headed over and had a shot saved before Adams brought a low save from Hamer at the other end but when the Bury wide man slipped the ball through to Bishop he made no mistake, lofting the ball over the Brentford goalkeeper and into the net.
The Bees continued to fight and Smith lashed an effort straight at Randolph from 25 yards but Bishop's second goal knocked the stuffing out of them and they slipped to defeat.
Brentford: Hamer; Mousinho, Sankofa (sub Shakes h/t), Osborne, Dickson; W. Brown (sub Reid h/t), Smith, Milsom, Poole; Connell, Elder
Subs (not used): Simon Brown, Charles, Pead
Bookings: Smith (28 mins) (fifth of season), Elder (41 mins) (second of season)
Bury: Randolph; Haslam, Futcher, Sodje, Buchanan; Bennett (sub Parrish 83 mins), Scott, Barry-Murphy, Adams; Hurst (sub Baker 75 mins), Bishop
Subs (not used): Provett, Rouse, Yeo
Bookings: Barry-Murphy (32 mins), Sodje (55 mins)
Attendance: 3,333 (139 away fans)
Written by Chris Wickham