Errors are handled in real channels by a combination of techniques and it is the overall result that matters. This means that different media and channels can have completely different approaches to the problem, yet still deliver reliable data.
In the data recording or transmission fields, any time a recovered bit is not the same as what was supplied to the channel, there has been an error. Different types of data have different tolerances to error. Any time the error rate is in excess of the tolerance, we need
Optical disks rely totally on the ability of the pickup to follow and focus on the data track. It is taken for granted that these mechanisms are phenomenally accurate, work at high speed despite being made at low cost and need no maintenance.