I recall this problem occuring in FreeBSD 5.3, if MPSAFE wasn't disabled for the network stack. However, I thought this bug was completely wiped out on 5.4.

[web02] ~# netstat -m
4294942564 mbufs in use
4294948752/25600 mbuf clusters in use (current/max)
0/22/6656 sfbufs in use (current/peak/max)
4151033 KBytes allocated to network
0 requests for sfbufs denied
0 requests for sfbufs delayed
5007 requests for I/O initiated by sendfile
208 calls to protocol drain routines
Oddly enough, it doesn't occur on all our 5.4 boxes, atleast not at this moment. They are all dual CPU, which from what I recall in 5.3 was related to the problem.

Anyway, anybody else still seeing this on their boxes?