In a few cases, the kernel will use pprint to put a diagnostic on the standard error (file descriptor 2). One of those is a warning that the process has used more than 100 file descriptors. That message is possibly obsolete and could be removed, but there are others, such as notifying an uncaught trap that are probably helpful to make visible. In any case, as things stand, a busy exportfs might have many file descriptors open, provoking the diagnostic. Unfortunately, aux/listen and aux/listen1 connect file descriptor 2 to the incoming network connection. If the connection's protocol is not a simple, unstructured, textual one, diagnostics on the standard error will cause confusion, in particular to devmnt.c if 9p is used.
/rc/bin/service files that start applications that run special protocols might want to redirect file descriptor 2; alternatively, perhaps aux/listen shouldn't redirect fd 2 by default: the few commands that do connect the remote user to shells, or equivalent, including telnetd and sshd could dup 1 to 2 when that was sensible.
