Eight days after crushing Northampton 5-2 in the FA Cup, Bignall's late header from Danny Jackman's corner gave the Cobblers a repeat of the scoreline of the only ever time they have previously beaten Leeds - back in 1966.
Northampton led in the 28th minute when a deflected cross by Ryan Gilligan fell into the path of Liam Davis and although he slipped, the midfielder rose to turn and fire a shot which David Lucas barely sniffed as it whistled past him.
That was the signal for Leeds to step up the pace, but they were denied by some heroic goalkeeping from Frank Fielding who turned away one goalbound effort from Jermaine Beckford before then denying the striker with his legs.
When Fielding couldn't get to a 38th minute blast from Fabian Delph, the ball ran to safety off a post.
Leeds continued to press with urgency after the break with Andy Robinson the chief tormentor down the right flank. He twice tested Fielding himself before delivering a 53rd minute cross which Luciano Becchio met with a snapshot which cannoned back off the crossbar.
Inevitably, Leeds drew level in the 69th minute when Beckford headed in a Robinson cross although Northampton felt Becchio had fouled Jason Crowe in the build-up.
All the late pressure continued to come from Leeds with shots from Becchio and Delph dealt with by Fielding, but the visitors were dealt a blow when Beckford limped off the field.
But the home side had saved their own version of 'tales of the unexpected' for the grand finale when the tiny Bignall headed in Jackman's right-wing corner.