The OpenBSD team released an Errata entry 007_tcprespond for OpenBSD 4.2.
Quote from the henning@’s commit message:
MFC (markus) when creating a response, use the correct TCP header instead of relying on the mbuf chain layout; with claudio@ and krw@; ok henning@
I’ve made binpatches for i386, amd64 and sparc64.