A 75th-minute blunder by goalkeeper Gorka Iraizoz handed Porto a 2-1 victory over Athletic Bilbao that helped the 1987 and 2004 winners maintain a vice-like grip on Champions League Group H.
Three minutes after coming on as a substitute, Ricardo Quaresma's right-foot shot from the edge of the area went straight at Iraizoz but the Spaniard somehow let the ball squirm under his body.
Porto had gone in front on the stroke of halftime when Mexican Hector Herrera produced an emphatic right-foot finish following a neat pass from Juan Quintero.
Bilbao equalised 13 minutes after the break when Guillermo Fernandez neatly sidestepped two defenders before expertly sliding the ball beyond Brazilian keeper Fabiano.
Several players squared up in a minor kerfuffle immediately after Guillermo's goal, with Porto's Danilo and Markel Susaeta of Bilbao being booked.
It was a quiet night for Porto striker Jackson Martinez who was unable to add to the three goals he scored in his side's opening two games.
The Portuguese team, who are second in their domestic championship, have seven points from three games and lead Group H by two points from Shakhtar Donetsk who romped to a 7-0 win at BATE Borisov thanks to five goals from Luiz Adriano.
Struggling Bilbao are languishing fourth from bottom in La Liga and are propping up the rest with one point in Group H, two behind BATE.
