Mansfield's menacing answer to Bobby Zamora inflicted Albion's worst defeat since the end of August when they beat the Seagulls 2-0.
All eyes were on Zamora again at Field Mill after Cardiff's rejected £1.2 million bid.
But Chris Greenacre upstaged the Seagulls' hot-shot with a clinical display of his own.
The former Manchester City forward stole the show with two second- half goals.
His superbly taken double leaves Micky Adams' men confronting an important test of character over the hectic Christmas period.
Adams anticipated a positive response to the FA Cup exit at Scunthorpe. In truth he didn't really get it.
His team gave a flat performance, enlivened only by the half-time introduction of Paul Brooker.
Adams' No.2 Bob Booker admitted: "Our foot was off the pedal a little bit, especially in the first half.
"We played in fits and starts. We had some good possession without an end product.
"We didn't create a lot of chances and things didn't quite fall for us."
Albion's away form in the League is beginning to cause concern. The last four games have yielded one win, two defeats, a draw and three blank sheets.
The same personnel and system has been producing opportunities and goals galore at Withdean, but that adventurous spirit is missing on their travels.
The players seem sub-consciously to have adopted a stance of trying to avoid defeat rather than striving for victory.
Adams indicated his dissatisfaction with the opening 45 minutes by bringing Brooker on at the expense of Nathan Jones and changing the formation.
Gary Hart was pushed inside to partner Zamora with Brooker given a free role behind the front pair, an experiment which proved effective against Cardiff in the LDV Vans Trophy.
Brooker injected some impetus into Albion's play and they had their best spell at the start of the second half.
Hart was unlucky not to give them the lead with a flick header from a Richard Carpenter free-kick which beat the competent Bobby Mimms, only for Stuart Hicks to somehow clear off the line.
Albion's Jekyll and Hyde nature home and away is emphasised by Zamora's scoring stats. All 14 of his League goals have been bagged at Withdean.
The Mansfield defence restricted him to one chance, a header straight at Mimms from a Kerry Mayo cross shortly after Albion fell behind.
The goal which ended a dour deadlock had an element of fortune about it.
Mark Blake's effort fell obligingly at the feet of Greenacre and he finished with aplomb.
A left-foot shot on the turn from virtually on the penalty spot sent Michel Kuipers the wrong way.
Kuipers was booked for immediately dashing towards the linesman. He claimed afterwards that Greenacre was three years offside, but Booker refused to make excuses.
"If the flag doesn't go up it's not offside, simple as that," he said.
With Albion pushing up in search of an equaliser, Greenacre warpped up only Mansfield's second victory in 12 matches a minute from time.
He controlled a diagonal pass from substitute Michael Boulding on his chest before calmly rounding the unprotected Kuipers.
Mansfield's recent record is misleading. Their home form is pretty formidable, Blackpool being the only side to come away from the impressivley refurbished Field Mill with three points.
The damage to Albion's promotion surge was also minimised by defeat for Chesterfield and a draw for Rochdale, while Darren Freeman's unexpected comeback offered further comfort.
Freeman, recovering rapidly from two hernia operations, came on down the right for the last 17 minutes, Adams withdrawing skipper Paul Rogers and adapting the system again to 4-4-2.
"Darren has been injured since I came here," added Booker. "He was quite bright and got some good crosses in.
"It's another player added to the squad which is important, especially with Christmas coming up."
Friday's visit by struggling Exeter launches a burst of six matches in 19 days, five of them in the League.
Three in just over a week in the New Year will examine the Withdean pitch, but in the context of their faltering away form it is the tough trips to Barnet and Rochdale which will test Albion's automatic promotion prospects.
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