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.