The home game against Hereford United had been flagged up as a must-win for Sampson during the week and a first-half Leon McKenzie hat-trick looked to have made that a formality for the Cobblers boss.
But he, like all the other home fans in the ground watched on in horror as League Two's bottom club netted four times without reply after the break.
Northampton, who ended the game reduced to ten men after Ben Tozer's dismissal, remain just outside the relegation zone but having squandered a 3-0 lead against the managerless Bulls, will come under pressure to make a change in the hot-seat themselves.
It was a frustrated and angry Sampson who spoke to the official club website after the game, saying: "I'm in total shock.
"We've let everyone down today and I'm lost for words as to how we can play so well in the first half and then so badly in the second half."