Ernesto Valverde's winning start as Valencia coach came to an end as striker Alejandro Dominguez's late penalty against his former club earned Rayo Vallecano a surprise 1-0 victory at the Mestalla.
Valverde had guided his new side to three successive away victories - one in the Copa del Rey, one in the Primera Division and one in the Champions League - without them conceding a goal since replacing Mauricio Pellegrini earlier this month.
But that fine run - and his home debut - was ruined by Dominguez as Valencia's league woes returned.
The 2002 and 2004 Spanish champions have not managed to string back-to-back Primera Division victories together this season and that sorry sequence continued thanks to Dominguez's 83rd-minute spot-kick, which was awarded following a mistake from Tino Costa and subsequent foul on Roberto Trashorras.
The result saw Rayo leapfrog their opponents into the top 10 and leaves Valencia closer to the bottom three than they are to top three.
Valencia's previous home match saw them thrashed 5-2 by Real Sociedad - their first Mestalla loss since April and one that spelled the end of Pellegrini's tenure - but supporters must have been hoping for a happier afternoon following Valverde's successful start.
However, it was to prove a day of disappointment for the home faithful as their side struggled to get going.
Visiting goalkeeper David Cobeno was barely troubled in the first half as Pablo Piatti, Jonas Oliveira, Andres Guardado and Ricardo Costa all failed to hit the target from openings.
Valencia goalkeeper Diego Alves was even more of a spectator before the break and it was a similar story after the interval with the first serious threat at either end coming in the 60th minute when Cobeno was called on to parry a shot from Guardado.
Rayo went close in the 80th minute when a shot by Trashorras deflected just wide off Costa, but with chances at a premium it seemed a goalless draw was the most likely outcome.
However, that changed with seven minutes remaining when Trashorras capitalised on a slip by Costa to steal possession and then went crashing just inside the area as the Valencia defender sought to muscle his opponent off the ball.
The penalty was awarded and Dominguez, who joined Rayo this summer after leaving Valencia, sent Alves the wrong way from 12 yards to earn all three points.