Corby Town came from behind at Butlin Road to scoop all three points and consign Valley to another spirited defeat.
Rugby had deservedly taken a lead into the half-time break after Micah Edwards marked his debut with a 26th minute strike, but The Steelmen put in a strong second half performance to prevail in the end.
Edwards and fellow debutant Jahvan Davidson-Miller were both drafted into Ian King's squad earlier in the week, and they started the game in place of Trey Charles and Madundo Semahimbo in two changes from the New Year's last-gasp defeat at Warwick.
Corby came into the fixture clear in second-place in the table, and they underlined their threat early on when Rory McAuley sent his header to Joe Butterworth's corner just wide of the target.
The hosts soon settled into the game too though, with Keiron Berry's tricky run into the box potentially halted illegally by Alfie Wren just before the quarter of an hour mark, but Rugby's appeals for a penalty were turned down by a well-placed match referee.
Harrison Nee had a shot blocked, before Butterworth drew out a comfortable save from Paul Hathaway at the other end.
Valley did then break the deadlock when Chris Clements' excellent long ball in behind the away defence was taken on by Edwards, and the midfielder skipped around keeper Ross Durrant to ultimately slot the ball home.
The visitors nearly grabbed themselves an equaliser shortly after, when Fletcher Toll lifted the ball into Reuben Marshall, but Hathaway spread himself well to keep out the resulting effort.
Rugby continued to impress though, and after Dan Summerfield had released Berry down the right-wing with a neatly-flighted pass, his dangerous delivery across the face of the goalmouth was cleared, with the next phase of play seeing Berry in again - only to be halted this time by the offside flag.
Town's two new signings then combined well to create another good opportunity, with Edwards's ball over the top of the Corby defence leading to a decent strike from Davidson-Miller - met by an equally good save from the legs of Durrant.
The last two chances of the half fell to the Northamptonshire team, with Spencer Edwards's volley deflected away for a corner and Danny Gordon heading wide a cross from Butterworth.
Rugby created the first opening of the new half, when Caine Elliott's great crossfield ball picked out Nee, but he could only blaze his effort high and wide of the frame.
However, Corby then visibly stepped up a gear or two after this early opportunity for Valley, and went on to dominate proceedings for a significant period.
McAuley had a header tipped over the bar by Hathaway in the middle of a spell of pressure and a series of corners from the visitors.
Summerfield then pulled off an amazing goal-line block with his head to deny Gordon's thunderbolt strike from a few yards out, with Hathaway also keeping a close range effort from Edwards.
The inevitable goal came from the spot on 68 minutes though, after Tyreace Brown was adjudged to have ended Kalern Thomas's marauding run into the box with an illegal challenge - with Toll duly rolling the penalty into the bottom right hand corner.
Ayo Lekuti's cross was blocked away after an interchange between Berry and Nee had created an opening down the left channel for him, but the visitors continued to look threatening, with Gordon's strike from distance touched over the bar by Hathaway.
Corby's second came on 76 minutes when Hathaway could only palm sub Tendai Daire's low shot into the path of Callum Milne, who applied a straightforward finish.
Thomas blazed one over after another determined run forward from the full-back, before Town finished the game strongly in search of a leveller.
Charles flashed a cross narrowly beyond the reach of Davidson-Miller, with Nee then denied by a good challenge at the back-post by Thomas after a further dangerous cross from Davidson-Miller.
Jordan O'Brien did lift one over for the visitors, before Dan Summerfield put in an off-target header to a Clements free-kick with the last contact of the game.
