esaProbeHistoryR2PErredPkts is a 64-bit performance counter counting test packets from the reflector back to the probe that failed to transmit because of local buffer/resource limits, for a completed interval that has already closed and been archived into the rolling history log; it does not apply to Y.1731 probes. A rising erred-packet count usually points to the probe itself running out of transmit buffers, which is a sign the configured packet rate is too aggressive for the platform rather than a fault in the network being tested. For example, finding this counter non-zero only in the archived bins right after a rate change was made explains, after the fact, why loss numbers briefly looked worse than the circuit really was.