Researches Chitti Nimmagadda and Dorr H. Clark of Santa Clara University seem to have discovered and reported a bug in usr/src/sys/fs/fifofs/fifo_vnops.c of FreeBSD 8.0-STABLE release as reported on the FreeBSD bugs mailinglist.
We believe we have identified a significant resource leak present in 6.x, 7.x, and 8.x. We believe this is a regression versus FreeBSD 4.x which appears to do the Right Thing ™.
We have a test program (see below) which will run the system out of sockets by repeated exercise of the failing code path in the kernel.
Our proposed fix is applied to the file usr/src/sys/fs/fifofs/fifo_vnops.c
If interested in (FreeBSD) code, have a look here for more info.