The Chiefs' defeat was made worse by the loss of Luke Jacobson, who managed just 11 minutes in his return from ongoing concussion issues before having to leave the game with what appeared to be a leg injury.
"We just didn't front up, particularly in that first half – we were pretty much still on the bus," said Chiefs captain Sam Cane.
Beaten after the hooter a week ago at home by the Highlanders, the Brumbies were at risk of seeing more competition points slipping through their hands after a dominant first half performance, as the Chiefs threatened constantly in the final half hour.
But for all the problems they caused the Brumbies, handling errors, issues at lineout time and some good work from the Brumbies at the breakdown kept the Chiefs from making the most of their territorial advantage.
The Brumbies took the fight to the Chiefs early and often, with pressure from the opening whistle culminating in a try to fullback Tom Banks inside the opening 10 minutes. He was followed over by former Warriors NRL star Solomone Kata five minutes later, and after 15 minutes the Chiefs faced a 12-point deficit.
Things got worse for the hosts just before halftime when No 8 Pete Samu crashed over from the back of a scrum, leading the Brumbies into the break with a 19-0 advantage.
It was a dismal 40 minutes for the Chiefs, and only got worse after halftime when Samu again crossed the line with ease. After 47 minutes, the Chiefs had missed 25 tackles - five times more than the Brumbies - and it showed on the scoreboard.
At that point, the Chiefs had to be the next to score if they had a hope of maintaining their perfect record, and did so in an unlikely fashion.
Enjoying a period of possession deep in Brumbies territory, it was first five-eighth Aaron Cruden running a hard line from five metres out to cut through the defence and score. Buoyed by finally getting on the board, the Chiefs were in again soon after, this time with Anton Lienert-Brown running the crash-ball line.
With 25 minutes remaining and the Chiefs needing two tries to match the visitors, they went searching for points and had plenty of opportunities but were let down by execution time after time, turning the ball over 17 times to the Brumbies' nine, giving the team and coaches plenty to think about heading into a bye week.
Brumbies 26 (Pete Samu 2, Tom Banks, Solomone Kata tries; Noah Lolesio 3 cons)
Chiefs 14 (Aaron Cruden, Anton Lienert-Brown tries; Damian McKenzie 2 cons)
HT: 19-0