Sheffield Wednesday substitute Anthony Musaba scored a late winner in the 89th minute, leading to Middlesbrough's 2-1 defeat at Hillsborough and denting their hopes of reaching the Sky Bet Championship play-offs.
Middlesbrough had the chance to climb into the top six after Finn Azaz opened the scoring in the 11th minute in a rainy South Yorkshire, but Josh Windass equalized early in the second half before Musaba secured Wednesday's first home victory since New Year's Day.
Middlesbrough missed an opportunity to increase their lead when Tommy Conway's penalty was saved by Owls goalkeeper Pierce Charles in the 22nd minute.
The win would have pushed Boro to sixth place based on goal difference, but they are now three points behind Coventry, who lost to Plymouth, with only two games remaining.
Middlesbrough manager Michael Carrick maintained the same starting lineup from the previous match, while Wednesday boss Danny Rohl made four changes following a 2-0 loss to Stoke.
Owls captain Barry Bannan tested keeper Mark Travers early on, but Boro took the lead through Azaz after a defensive error from Wednesday's Marvin Johnson.
Boro had a chance to score again from the penalty spot, but Conway's effort was saved by Charles.
Wednesday's slim play-off hopes suffered a blow amidst a difficult period, marked by poor results, unpaid players, uncertainty around the coach's future, and fan protests against the chairman.
Windass and Charles played crucial roles in keeping Wednesday in the game, with Windass eventually equalizing in the second half.
After Boro threatened to regain the lead, it was Musaba who secured the victory for Wednesday in the final minute.
Charles made a decisive save late in the game to prevent Boro from equalizing, ensuring Wednesday's long-awaited win at home.