Albion suffered a heartbreaking end to their ten-match unbeaten run against the Third Division leaders.
Chesterfield snatched a victory they scarcely deserved with a 91st-minute goal by substitute Jamie Ingledow after the Seagulls had played most of the second half with ten men.
Ingledow, brought on in place of Jon Howard in the 82nd minute, headed in a cross from Ryan Williams to break Albion's magnificent resistance.
The turning point of a hard-fought contest was the 51st minute dismissal of Charlie Oatway for a second bookable offence.
Oatway was cautioned in first-half stoppage time for retaliating to a challenge by Williams.
A red card became inevitable for the pugnacious midfielder when he tripped Sean Parrish in the centre circle.
Boss Micky Adams reshuffled, bringing on Andy Crosby as an extra central defender among three second-half changes.
Matthew Wicks was immaculate yet again and Michel Kuipers made two fine saves to foil Marcus Ebdon.
Albion looked like holding out until Ingledow's last-gasp sickener opened a seven-point gap between the sides.
The Seagulls had been the better team up until Oatway's sending off.
Bobby Zamora shot just wide in the first half and Richard Carpenter had another effort pushed over by keeper Mike Pollitt.
The kick-off was delayed for 15 minutes by Preston referee Mike Ryan after some of the 2,400 Albion fans in a crowd of 7,014 were delayed by a motorway accident.
A mindless minority reacted badly to Albion's unfortunate defeat, hurling missiles at stewards as they left.
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