An optical sensor system may include an interrogation system coupled to one or more fiber-optic cables positioned in a wellbore. The interrogation system may interrogate a fiber Fabry-Perot interferometric sensor and one or more fiber Bragg sensors multiplexed in the fiber-optic cables. The optical sensor system may include a processing device to determine whether there is condition that may cause an error in the calculation of the cavity length of the fiber Fabry-Perot interferometric sensor in real-time based on a reference distance between peaks or valleys in the reflected light signal generated by the fiber Fabry-Perot interferometric sensor. The processing device may compare the reference distance to a distance between peaks or valleys detected in the spectrum to identify that an error condition exists and may correct the error prior to calculating the cavity length.