FreeBSD kernel 4.6 and earlier closes the file descriptors 0, 1, and 2 after they have already been assigned to /dev/null when the descriptors reference procfs or linprocfs, which could allow local users to reuse the file descriptors in a setuid or setgid program to modify critical data and gain privileges.
The kqueue mechanism in FreeBSD 4.3 through 4.6 STABLE allows local users to cause a denial of service (kernel panic) via a pipe call in which one end is terminated and an EVFILT_WRITE filter is registered for the other end.