Northampton suffered their first home defeat in more than four months as they were beaten 1-0 by promotion rivals Port Vale.
Leyton Orient were the last visiting team to win at Sixfields on October 5, but Vale repeated that feat thanks to David Worrall's second-half header.
Worrall shot wide in the first few minutes, while Scott Brown made two comfortable stops at the other end, keeping out Mark Marshall and Nicky Adams.
Lloyd Jones also had a shot blocked, but a dour first half was otherwise short on clear-cut chances.
Callum Morton found the side-netting moments after the resumption and Adams then missed a golden opportunity, side-footing wide when left unmarked.
Brown tipped over Sam Hoskins' volley and reacted superbly to thwart Vadaine Oliver as the Cobblers cranked it up.
But it was the visitors who took a shock lead with 18 minutes to go, when Tom Conlon's cross was headed home by Worrall at the back post.
Northampton piled on the pressure in search of an equaliser, but Brown continued to frustrate them, saving excellently from Adams late on.