The Lions lost their unbeaten tour record just four days before tackling Australia after Jake White's Brumbies deservedly triumphed 14-12 at Canberra Stadium.
In doing so, they became the first Australian provincial side since Queensland 42 years ago to overturn the Lions.
And there could be few realistic complaints from the visitors, who were outgunned up front for an hour before coach Warren Gatland summoned reinforcements off the bench.
Centre Tevita Kuridrani struck early for the game's only try, while Brumbies full-back Jesse Mogg kicked three penalties.
The Lions, who were always behind, managed two Stuart Hogg penalties and two Owen Farrell strikes.
Hogg twice saw penalties hit the post, but a new-look three-quarter line of Christian Wade, Brad Barritt, Billy Twelvetrees and 36-year-old Shane Williams featured in a team that looked devoid of quality preparation time.
The backs were powerless at times, having to scamper and retreat as a disorganised and mistake-riddled Lions pack performance helped the Brumbies eight dominate.
Gatland will not be alarmed ahead of the first Test in Brisbane - his Test players were wrapped in cotton wool today - but hopes of an invincible tour are now over.
They fell behind inside five minutes after Williams - the oldest back to play for the Lions in history - had an early chance to show his pace, but the Brumbies cleared and put together a sustained passage of attacking play.
And it ended with Kuridrani barging through the attempted challenge of Wade and Rob Kearney to score a try that underlined how big a task the Lions faced.
It was the first time the Lions had been behind in a game since they trailed Queensland Reds 10 days ago and they were not helped by a malfunctioning lineout, with skipper Rory Best's erratic throwing a problem.
The Lions should have opened their account midway through the half, but Hogg saw a straightforward penalty chance bounce back off a post and the Brumbies escaped.
The Brumbies were content to play things tight and use their ball-carrying forwards, although the Lions had a fleeting opportunity when Williams jinked clear but was harshly ruled to have had a foot in touch.
Mogg then sent a 52-metre penalty just wide after Lions prop Ryan Grant was whistled by French referee Jerome Garces for offside, and the Lions still could not get going.
The forwards continued to struggle for quality possession, which meant a difficult opening 40 minutes for half-backs Hogg and Ben Youngs, and a first defeat on tour beckoned unless they could raise their game.
But Mogg made it 8-0 with a penalty on the stroke of half-time and despite Hogg responding, the Lions could have few complaints.
They began the second period with a greater sense of purpose and it took some organised defence to keep him out, but then the Lions' familiar problems surfaced.
A botched lineout led to a Brumbies break-out and when flanker Sean O'Brien was penalised for not rolling away, another successful Mogg kick restored an eight-point advantage.
The Lions continued to concede a ridiculous number of turnovers and with their set-piece not functioning, there was very little a hastily-assembled back division could do.
Grant conceded another penalty, this time for foul play, and Mogg completed his penalty hat-trick before a second Hogg strike kept the Lions in it.
Changes that had long been expected arrived midway through the second half when Gatland sent on a new front row of Alex Corbisiero, Richard Hibbard and Dan Cole, with flanker Dan Lydiate replacing O'Brien.
The Lions desperately needed some impetus after misfiring for much of the game, and Gatland decided with 18 minutes remaining that it was time for Farrell to replace Hogg as playmaker.
And his first contribition was to kick an angled penalty that brought the Lions back within a try of levelling up a game they had played second fiddle in for so long.
Another Farrell penalty made it 14-12, setting up a tense final nine minutes as the Brumbies tired and the Lions substitutes continued upping the ante, but it was ultimately all to no avail.
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