Sampdoria went ahead two minutes after half time when Inter defender Ivan Cordoba fouled striker Francesco Flachi, the 31 year old regaining his composure before converting the spot kick.
The goal was against the run of play and Inter continued to search for an equaliser.
The Nerazzurri were frustrated on 71 minutes when former Juventus midfielder Patrick Vieira found the back of the net only to be incorrectly ruled off side.
Sampdoria goalkeeper Luca Castellazzi was looking like securing his side's victory with a string of fine saves, but with 11 minutes remaining Blucerchiati midfielder Massimo Bonanni header the ball into his own net from a corner and the howler meant the points would be shared.
Earlier on Saturday, Reggina striker Rolando Bianchi scored his fourth goal of the season when he slammed home a stoppage time winner to earn his side a 2-1 win over Cagliari.
It would have been a satisfying strike for the 23 year old whose hat-trick last weekend couldn't prevent his side being defeated 4-3 by Palermo.