If the timing of a "seqno-ack" from consumer to producer is just right, it can find that the active vbucket has been deleted, the producer will respond to the consumer with "not-my-vbucket". The consumer will receive the response packet and error as follows:
WARNING 121: Unsupported response packet received with opcode: 0x61 (DCP_SEQNO_ACKNOWLEDGED)
The error happens because "mcbp" has no response handler configured for this opcode. This forces a disconnect of replication, which will fail a rebalance.
Clearly from the code, a seqno-ack can generate a response and we should gracefully handle them to avoid unexpected disconnects and rebalance failure.