This counter, in blocks, tallies how many FEC blocks on an OTUC2 signal arrived with errors the FEC decoder could not correct. It matters to an operator because uncorrected blocks translate directly into corrupted payload data reaching the client signal, unlike the corrected-error counters that represent errors FEC already fixed. For instance, a technician troubleshooting sporadic client-side traffic errors on a 200G circuit would check this counter to confirm the optical layer, not the router, is the source.