Napoli were seeking a victory that would have seen them close to within four points of second-placed Roma but Reina was at fault as he fumbled the ball into his own net from Ibrahima Mbaye's tame effort.
The goalkeeper's horror moment arrived just seven minutes after Dries Mertens had fired the visitors into the lead from the penalty spot, as Rafael Benitez's men were held to a frustrating 1-1 draw at Stadio Armando Picchi.
Napoli came into the contest on a high after their midweek Europa League win over Swansea and nearly went ahead in the 11th minute when Mbaye slammed the ball against his own bar attempting to dispossess Marek Hamsik.
The visitors from Naples were without top-scorer Gonzalo Higuain due to suspension but still were asking plenty of questions of the Livorno defence, with Mertens soon drawing a good save from Francesco Bardi.
Jose Callejon flashed one wide as Napoli continued to press and with 32 minutes gone the visitors had their reward when Federico Ceccherini pushed Goran Pandev in the area and a penalty was awarded, from which Mertens tucked in the opener.
The hosts were soon on level terms, although they needed a big gift from Reina.
Mbaye was the beneficiary of the Spaniard's error, his flick from Djamel Mesbah's cross lacking any real power but somehow slipping from Reina's grasp and into the net.
Reina partially atoned for that moment by saving from Paulinho in the second half as Livorno pushed for a winner, but Napoli continued to offer menace up front with Hamsik seeing a header well saved by Bardi, who also went on to deny Callejon inside the final 10 minutes.
Napoli very nearly spared Reina's blushes in the final minute when Lorenzo Insigne's fine run ended with a low centre across the face of goal but fellow substitute Duvan Zapata somehow failed to get a telling touch.