I have the following network (simplified of course):
  +---+ 10.0.3.67/24    +---+ 10.0.0.180/24    +---+

  | A +-----------------+ B +------------------+ C +

  +---+   10.0.3.180/24 +---+     10.0.0.34/24 +---+
A and C are Linux hosts, while B is running OpenBSD 4.2.  B has

net.inet.ip.forwarding=1, and A and C are normally able to

communicate.  B is running pf with a state table of about 200K entries

with a limit of 800K, but none of the rules affect A<->C.
However, occasionally I see bursts of errors where when A tries to

establish a TCP connection to C, that B returns an "ICMP host

unreachable" error.  I see long strings of these even while both A and

B are able to successfully ping C.  E.g., here's a short snippet from

running tcpdump on A just a few minutes ago:
11:18:25.039132 IP 10.0.3.67 > 10.0.0.34: ICMP echo request, id 65358,

seq 10, length 64

11:18:25.039484 IP 10.0.0.34 > 10.0.3.67: ICMP echo reply, id 65358,

seq 10, length 64

11:18:25.110883 IP 10.0.3.180 > 10.0.3.67: ICMP host 10.0.0.34

unreachable, length 36

11:18:26.038676 IP 10.0.3.67 > 10.0.0.34: ICMP echo request, id 65358,

seq 11, length 64

11:18:26.038981 IP 10.0.0.34 > 10.0.3.67: ICMP echo reply, id 65358,

seq 11, length 64
Why would B send these host unreachable errors when the host is

clearly reachable?
Thanks.