the only local diff is a NAT which checks every incoming packet for NAT routing before it is passed to LwIP (in case it's not rewritten). But that's not applied to local connections from LwIP (which we have here).
Any debugs which would help here? It would be hard to anyone to reproduce this via a fast Ethernet link as the xmit is too fast here to have the first packet still in transmit while the second one is written. With the slow PPP link I can 100% reproduce it.
Vanilla 1.4.1? Absolutely no local diffs?
Simon