Third-placed Napoli had taken a first-half lead with Gonzalo Higuain's 13th Serie A goal this season before Calaio curled a left-foot free kick over the wall and in off the post with six minutes left.
Calaio, who joined Napoli when they were in Serie C in 2005 and helped them return to the top flight, was muted in his celebrations.
"I don't like scoring against Napoli, where I had a significant phase of my career, but this is my job and my career," he said.
Calaio left Napoli in 2008 but returned last year before being loaned to Genoa, who have an option to buy him, this season.
Napoli are six points behind second-placed Roma after continuing their frustrating habit of dropping points against mid-table opponents.
Rafael Benitez's side, who occupy the Champions League play-off place, have 51 points, six clear of fourth-placed Fiorentina, who earlier drew 2-2 at Parma in a game which produced three red cards.
Roma, with a game in hand over the other teams in the top four, have 57 points and leaders Juventus 66.
Fiore's Matias Fernandez curled in a majestic 30-yard free kick five minutes from time to earn them a point at Parma.
The Chilean sent an ambitious, diagonal shot over the wall into the far top corner as they twice came from behind despite playing most of the second half with 10 men.
Parma, who are seventh on 37 points, regained the lead six minutes after halftime with a controversial penalty awarded for a high challenge by Nenad Tomovic on Jonathan Biabany and converted by Amauri.
Fiore, already seething over that decision, were further aggrieved two minutes later when Modibo Diakite was sent off for a second bookable offence after a foul on Cassano.
There was more trouble in stoppage time when Parma were awarded a free-kick outside the Fiorentina penalty area and a scuffle broke out as the visitors tried to organise their wall.
Fiorentina playmaker Borja Valero and Parma substitute Gianni Munari, who had only been on the field for 10 minutes, were sent off for lashing out.
"I always tell the team to control these situations, but perhaps those who come from different leagues struggle to deal with the tension," Fiore coach Vincenzo Montella said.
"The team is a little irritable and that's understandable when they feel that a few incidents have gone against them."