On a path with high latency and high retransmits, you see this:

./iperf3 -w64M -t20 -i1 -c newy-pt1 Connecting to host newy-pt1, port 5201 [ 4] local 198.129.254.150 port 34137 connected to 198.124.238.54 port 5201 [ ID] Interval Transfer Bandwidth Retransmits [ 4] 0.00-1.00 sec 76.2 MBytes 640 Mbits/sec 4 [ 4] 1.00-2.00 sec 0.00 Bytes 0.00 bits/sec 2 iperf3: error: bytes <= 0! [ 4] 2.00-3.00 sec 1.25 MBytes 10.5 Mbits/sec 2 [ 4] 3.00-4.00 sec 0.00 Bytes 0.00 bits/sec 2 iperf3: error: bytes <= 0! [ 4] 4.00-5.00 sec 1.25 MBytes 10.5 Mbits/sec 0 [ 4] 5.00-6.00 sec 1.25 MBytes 10.5 Mbits/sec 2 [ 4] 6.00-7.00 sec 1.25 MBytes 10.5 Mbits/sec 1 [ 4] 7.00-8.00 sec 1.25 MBytes 10.5 Mbits/sec 1

No need to print this error, as its "normal" for this situation.

I assume bytes < 0 should still be notable.