When an audio disc is read with enough consecutive bits incorrect (flagged by the CIRC circuitry of the reading drive), resulting in a CU error, concealment happens.
Concealment is the process of trying to 'hide' the loss of data in the audio bitstream and usually interpolating (basically guessing based on previous known data) the samples that are flagged as incorrect.
If the interpolation works well (esp. with higher order interpolation) and the loss of data was not big, then you may not hear a difference in the audio stream at all.
However the error has still happened for that particular reader.
Please remember also that each drive's abilitity to track and correctly extract bits off a disc is unique. Some do it better than others.
Futhermore, after the bits are read off the disc, they are passed through the error correction circuitry. The performance of this circuitry may also vary and some drives do it better than others.
Finally, the concealment of uncorrectable errors is where a lot of differences manifest between various drives. Some drives just mute the signal when uncorrectable errors happen while other drives interpolate using various functions.
So, with one disc you might get an uncorrectable error in drive A, but not in drive B, whereas you might not hear the error in drive B, but you will hear it in drive C (usually as a pop/click or loss of sound for a very short time).