Valley's winning streak was brought to an abrupt end as visitors Desborough Town scored a stoppage time winner to take the points.
Substitute Iain Blaikie's header at the death meant a below-par on the night Rugby missed out on the chance to establish a club record for consecutive league victories and had to settle for a share of it with thirteen instead.
An unchanged Town starting eleven created the first chance of the game, but David Kolodynski blazed over Caine Elliott's pass. Desborough's Archie Elmore spurned an even better opportunity when he missed the target after Simeone Okoro had got in behind the Valley defence to set up the striker.
Away keeper Adam Honour was then down well to keep out a Liam Francis header, with Elliott also seeing his effort from the edge of the penalty area comfortably taken by Honour.
Elliott lifted a further chance over the bar from a dangerous position after Kolodynski had hurried the busy keeper into a half clearance, before Alex Lock flashed a shot across the face of the goalmouth only to see Liam Honour deflect the ball away for a corner.
Keenah Rosser's pinpoint pass picked out Justin Marsden in space in the box, but he was just unable to put in a meaningful strike, with Lock then forcing an acrobatic save from Honour as Rugby continued to search for a breakthrough.
Edwin Ahenkorah's low effort drifted just wide, ahead of the visitors' best spell of the half which saw Matt Hill at full length to deny the dangerous-looking Okoro and then Callum Ballinger send one wide.
Okoro also whipped-in a teasing cross just beyond his team-mates and Lock sliced a further shot at the other end, before the two teams went off for the break with all still to play for.
There was a scrappy, uneventful start to the second half, although Elliott nearly capitalised on Ahenkorah's deep cross to the back post after Town sub Madundo Semahimbo had done well to set him free down the left wing.
Semahimbo's strike to Elliott's pass was deflected up onto the bar, before Desborough reminded the hosts of their threat when Liam Boath fired one over the target and shortly after this Rosser also did well to block away Alvin Magagada's attempted shot on goal.
Rugby's strong appeal for a penalty after Kolodynski's shot to a Drew Kear centre was turned down by the referee, with Hill taking Rico Duggan's effort as Desborough continued to threaten occasionally on the counterattack.
Honour pulled off his most impressive stop of the game by responding well to a deflection to a Semahimbo strike to push the ball away for a corner, before making another good save to Elliott's low shot after the midfielder had cut inside from the left.
Valley turned the screw in the closing stages as they pushed for the record-breaking win, with Honour again in the action in catching Ahenkorah's rising effort and then Kear flashing one wide from an angled position.
A further decent shout for a penalty was waived away when Elliott appeared to have been tugged down in the goalmouth after Loyiso Recci had chipped the ball into a crowded box, before Desborough struck their killer blow in the 93rd minute when Blaikie nodded in Lewis McLean's corner to the near post.
There was still time for Josh Thomas to blaze one way over as Valley looked to salvage a point from the encounter, but alas it was not to be and their long unbeaten run was finally over.
Valley Line-up: Hill; Rosser, Francis, Recci; English (Kear), Ahenkorah, Lock (Seal), Fitzharris (Thomas), Elliott; Marsden (Semahimbo), Kolodynski. Subs not used: Young. Booked: None.
