Saprissa had won five matches in succession - six in all competitions - prior to the weekend, but were humbled by lowly Belen Siglo XXI 4-3 on Sunday.
Jeaustin Campos' men appeared on track to preserve their winning run despite conceding in the second minute.
Mario Centeno put Belen Siglo XXI ahead early, but the Estadio Eladio Rosabal Cordero hosts were trailing 16 minutes later after goals to Saprissa's Keilor Soto and David Ramirez.
But Belen Siglo XXI's response was emphatic - Leonardo Adams equalising prior to half-time before a six-minute cameo from Jose Luis Cordero gave them a match-winning two-goal lead.
Ramirez's second of the match with six minutes of regulation gave the visitors hope, but they fell to their fourth defeat of the campaign.
Saprissa remain in third place, three points behind Alajuelense - who suffered defeat for just the second time as they went down 1-0 at Herediano.
Alajuelense were not made to pay for their slip up, though, as UCR - second, two points back - and fourth-placed Cartagines (27) also failed to win.
Cartagines struck late against UCR to secure a 1-1 draw, Andres Lezcano scoring a 90th-minute equaliser.
Cellar dwellers Uruguay earned their first win of the season, Jonathan Moya's penalty on the stroke of half-time helping them beat second bottom Puma Generalena 1-0.
Limon strung together back-to-back wins, beating Santos de Guapiles 2-0 away from home to move within two points of the eighth-placed side.
Perez Zeledon are without a win in three, and had to come from behind to draw 1-1 at home to Carmelita.
Get the best features, fun and footballing frolics straight to your inbox every week.
Thank you for signing up to Four Four Two. You will receive a verification email shortly.
There was a problem. Please refresh the page and try again.