Bob McHugh came off the bench to score a superb winner for Motherwell and heap more misery on Aberdeen.
The home fans had booed the decision to take off striker Jamie Murphy 15 minutes from time.
But replacement McHugh fired a long-range drive past goalkeeper David Gonzalez nine minutes later.
Scott Vernon had a first-half shot saved at the other end, but the Dons rarely threatened and Well's Chris Humphrey also struck the woodwork.
Aberdeen manager Craig Brown had made four changes in the wake of the midweek League Cup humbling by East Fife.
Youl Mawene and Rob Milsom returned from injury, while Rory Fallon and Isaac Osbourne were promoted from the bench, with Darren Mackie, Rory McArdle, Peter Pawlett and Josh Magennis dropping out.
Third-top Motherwell were unchanged for a third successive game despite their defeats by Celtic, St Johnstone and Hibernian, the latter a penalty shoot-out exit in the League Cup.
But they looked determined to change that run from kick-off and, within seconds, Murphy had sent a drive just wide of a post after a fine run deep into the Dons half.
The game did not live up to that lively opening and Aberdeen took another 15 minutes to threaten, with Vernon forcing goalkeeper Darren Randolph to turn the ball wide with a strong hand as the striker tried to find the bottom corner with a low drive.
Motherwell squandered a great chance when Humphrey raced clear of marker Andrew Considine and rounded Gonzalez.
However, the winger opted for power instead of a simple side-foot into the net and sent the ball crashing off the crossbar.
Motherwell struck woodwork again immediately after the break when a cleverly disguised free-kick from Tom Hateley found Tim Clancy, but the defender's header powered against the crossbar.
The game appeared to be petering out until McHugh, on as a substitute after replacing Murphy, won the ball himself off Ricky Foster before unleashing a superb drive past Gonzalez.