This counts ATM cells received with a failed Header Error Control (HEC) checksum, meaning the cell header itself was corrupted in transit. Because HEC specifically protects the cell header, which carries the VPI and VCI routing information, a rising count here points to line-level bit errors severe enough to corrupt cell headers, a more serious signal than payload-only errors, since a header failure means the cell can't even be routed to the right circuit and gets discarded outright. For example, cardInErrorCells climbing on one port after a fiber patch panel was disturbed suggests a marginal physical connection introducing header-corrupting bit errors on that specific link.