With Juventus drawing 0-0 with Lazio on Saturday, second-placed Inter could have moved to within two points of the pacesetters with a victory on Sunday.
However, they not only failed to win, but they needed an 82nd-minute Davide Astori own goal just to salvage a point after a Marco Sau brace had seen impressive Cagliari bounce back from an early deficit to take the lead.
Inter had seen their six-match Serie A winning streak ended by last week's 3-2 loss at Atalanta and their hopes of getting back on track on Sunday looked good when Rodrigo Palacio headed them in front after just 10 minutes.
However, Inter struggled to build on that and it was Cagliari who looked the much more dangerous in the remainder of the first half.
Inter goalkeeper Samir Handanovic had already been called into action even before Palacio's goal to make a good save to keep out an Anderson Nene shot and the home side were indebted to a vital intervention from Walter Samuel to deny Nene again in the 13th minute.
The unmarked Nene had another opportunity 10 minutes later following good work by Radja Nainggolan but the Brazilian forward fired wide from 12 yards out.
Nene was proving a real thorn in Inter's side and it needed a miraculous save from Handanovic to prevent him equalising from close range in the 29th minute, while three minutes later the Inter keeper came out on top in the personal battle between the two with another fine save from the former Paris St Germain player.
Cagliari finally got the goal they had been threatening in the 43rd minute, and for once Nene was not the man involved.
A great left-wing cross from Danilo Avelar picked out Sau and he got in between Samuel and Andrea Ranocchia to slide the ball in on the half-volley.
Inter finally created another chance of their own at the start of the second half but Diego Milito wasted a gilt-edge chance as he fired over from close range with the goal at his mercy.
Milito did better just before the hour but his shot from the edge of the box was well-saved by Michale Agazzi.
Inter then found themselves trailing in the 66th minute when Sau slotted home his second of the afternoon after Maurizio Pinilla's acrobatic effort came back off the post.
That looked set to earn Cagliari all three points but the visitors shot themselves in the foot when the sliding Astori, in attempting to block Ricardo Alvarez's cross at the near post, succeeded only in diverting the ball into his own net.
Inter had two chances to snatch the three points in the closing stages but Agazzi was on top form to keep out efforts from Ranocchia and Philippe Coutinho.