The Brumbies missed the chance to move into second place in the Super Rugby standings when they lost 21-15 to the Force in Perth.
The Brumbies had already won the Australian Super Rugby Conference and a victory could have moved them provisionally into second place in the standings.
As it stands now the Brumbies will travel home to Canberra next weekend to play the Cheetahs on Sunday.
The result means that the Bulls will finish in the top two and will get next weekend off and a home Super Rugby Semi-final regardless of whether or not they beat the Stormers.
Anchored near the bottom of the ladder, the unfancied Force scored the first three tries of the match to post one of the major upsets of the year, notching just their fourth win to finish a tough season on a high note.
Fortunately for the Brumbies, they had already booked their berth in the playoffs so the result did not have any significant repercussions, aside from dented confidence heading into the business end of the season.
It was a bad night for the visitors from the very outset, as they put the ball into touch on the full from the kickoff in a portent of what was to come.
They looked to have a slim chance of stealing the win when the Force's Jayden Hayward missed a penalty in the 80th minute, but Matt Toomua inexplicably forgot to ground the ball before passing it forwards to a teammate to resume play.
That handed a scrum feed to the grateful Force, who had two potential tries turned down by the video referee, under the posts and they kicked the ball out as soon as they gained possession.
Brumbies captain Ben Mowen said the result was a timely wake-up call for the Brumbies.
"We didn't play good territory all game and our kicking really let us down," he said.
"The Force were extremely enthusiastic and played the conditions well.
"They played smart footy and exposed us.
"It is probably a really good wake-up call for us going into a finals campaign.
"We are fortunate we have another chance to play next week."
Mowen said the Brumbies still believed they could win the Super 15 title.
Force skipper Matt Hodgson was thrilled his team finished the season positively, especially with some players turning out for the team for the last time.
"We put it in for the full 80 minutes tonight and we had a belief," he said.
"We had nothing to lose and we wanted to finish on a high note."
The Force started superbly, with Hodgson and Ben McCalman running in first-half tries as the home side took a 14-3 lead into the break.
They extended that lead in the 47th minute, when Junior Rasolea capitalised on a line break from Nick Cummins to score his first Super Rugby try.
The Jayden Hayward conversion had them ahead 21-3 and they looked in total control.
However, the Brumbies struck back through tries to Tevita Kuridrani and Scott Fardy to narrow the deficit to 21-15 in the 66th minute.
With Hugh McMeniman stretchered off, the Force looked vulnerable, but were able to hold the Brumbies at bay as the visitors paid for their handling errors.
Final Score Western Force 21 (14) Brumbies 15 (3)
Tries - M.Hodgson, B.McCalman, J. Rasolea
Con - J.Hayward 3
Tries - T.Kuridrani, S. Fardy
Pen - C.Lealiifano
Con - C.Lealiifano
Referee: Rohan Hoffmann
Assistant Ref 1:Ian Smith
Assistant Ref 2:Damien Mitchelmore
TMO : Matt Goddard
1. Pek Cowan, 2. James Hilterbrand, 3. Salesi Ma'afu, 4. Sam Wykes, 5. Hugh McMeniman, 6. Matt Hodgson (c), 7.Chris Alcock, 8. Ben McCalman, 9. Alby Mathewson, 10. Sias Ebersohn, 11. Nick Cummins, 12. Junior Rasolea, 13.Ben Jacobs, 14. Patrick Dellit, 15. Jayden Hayward.
Replacements: 16. Ben Whittaker, 17. Salesi Manu, 18. Toby Lynn, 19. Richard Brown, 20. Brett Sheehan, 21. Sam Norton-Knight, 22. Sam Christie.
1. Scott Sio,2. Stephen Moore,3. Ben Alexander,4. Scott Fardy,5. Sam Carter,6. Peter Kimlin,7. Colby Faingaa,8. Ben Mowen,9. Ian Prior,10. Matt Toomua,11. Joe Tomane,12. Christian Lealiifano,13. Tevita Kuridrani,14. Henry Speight,15. Jesse Mogg
Replacements:16. Siliva Siliva,17. Ruan Smith,18. Fotu Auelua,19. George Smith,20. Robbie Coleman,21. Andrew Smith,22. Mark Swanepoel.