It's around line 301 of sys/kern/uipc_usrreq.c in the unix domain
socket implementation.

The immediately surrounding context is that upper layers have decided
to call uipc_usrreq with a request of PRU_SEND on a unix domain stream
socket, but the socket isn't connected, so it panics because it
doesn't know where to send the data.  (if the socket wasn't connected,
it shouldn't have gotten that far..)

It would be good to know what provoked it.

				- Bill