Paraguay striker Jorge Achucarro headed the winning goal as Argentine title holders Estudiantes were upset 1-0 at home by Banfield at the Ciudad de La Plata stadium on Sunday.
Estudiantes, still trying to settle into a more attacking style sought by new coach Eduardo Berizzo, were exposed in a quick counter-attack on the hour.
Midfielder Marcelo Carrusca crossed high from the left and Achucarro rose between two defenders to head past a static Agustin Orion in the Estudiantes goal.
Estudiantes, winners of the Apertura championship in the first half of the season, have six points from three matches in the Clausura. Banfield have four after their first win.
The top of the table has an unfamiliar look with Olimpo in the lead on seven points, one more than Lanus, Racing Club and Estudiantes. River Plate could join Olimpo with a win at Independiente later on Sunday.
Boca Juniors' poor start to the season at their Bombonera (chocolate box) home continued when they were held 0-0 by All Boys without dropped playmaker Juan Roman Riquelme on Saturday.
Coach Julio Cesar Falcioni believed Riquelme's stop-start style in midfield unbalanced his side and sought extra fitness work from the fans' idol while he fielded his favourite 4-4-2 line-up with two holding midfielders.
Falcioni, whose team had lost their opening match 4-1 at home with Riquelme and won 1-0 at Racing Club last weekend while he was out injured, was slated by fans who chanted Riquelme's name throughout the match.
Racing went from the despair of losing their Colombia playmaker Giovanni Moreno for six months with damaged knee ligaments two weeks ago to renewed hope in compatriot Teofilo Gutierrez's two-goal performance on Saturday.
Newly-signed striker Gutierrez scored with a diving header on the stroke of halftime and a close-range volley as Miguel Angel Russo's side came from a goal down to win 2-1 away to San Lorenzo.