Two games in three second-half minutes from Carlisle United saw them to a surprise win at Sixfields on Tuesday night.
Another hard-fought home game for Northampton was settled by strikes from Kennedy and Dempsey to leave the hosts four points away from the play-off places.
Visiting boss Keith Curle felt his side had a 'little bit of luck' at times during the game but that they had earned a break for sticking to their game plan.
"We kept our shape well after the break and we were disciplined in the way we played," he told the official CUFC website.
Northampton now travel to Bury on Saturday afternoon hoping to keep alive their hopes of a top-seven finish after a night on which they victims, perhaps, of their own raised expectations.
Chris Wilder accepted that the visiting side were happy to sit deep and play on the counter-attack but admitted afterwards that Northampton did not play with their 'usual zip'.