Today is the first time I've seen this; and I reproduced it three times in
a row, sitting at a Windows 98 Second Edition client behind a NetBSD NAT.

Here's what I did:

Using CuteFTP 4.2 beta on the Windows box, I FTP'd through the NAT to a
friend's Windows box down the road, IP address 10.160.61.178. When I
connect to it from the NAT itself, I get the following response:

blackhole:~$ ftp 10.160.61.178
Connected to 10.160.61.178.
220-Wolfpac Industries FTP service
    WarFTPd 1.70.b01.04 (Aug 18 1998) Ready
    (C)opyright 1996 - 1998 by Jarle (jgaa) Aase - all rights reserved.
220 Please enter your user name.
Name (10.160.61.178:jmcneill): 

However, whenever I connect to it through the NAT with CuteFTP, the NAT
locks. I haven't tested with any other clients; I don't enjoy pressing the
reset button every few minutes.

NAT information:

NetBSD blackhole.invisible.ca 1.5E NetBSD 1.5E (BLACKHOLE) #0: Sun Aug 27
05:12:33 ADT 2000
jmcneill@sun:/usr/src/sys/arch/i386/compile/BLACKHOLE i386

ep0 at isa0 port 0x300-0x30f irq 10: 3Com 3C509 Ethernet
ep0: address 00:20:af:70:1a:7c, 8KB byte-wide FIFO, 5:3 Rx:Tx split
ep0: 10baseT, 10base5 (default 10baseT)
ep1 at isapnp0 port 0x210/16 irq 3
ep1: 3Com 3C509B EtherLink III
ep1: address 00:20:af:70:1f:08, 8KB byte-wide FIFO, 5:3 Rx:Tx split
ep1: 10baseT, 10base5 (default 10baseT)

IP Filter: v3.4.9 initialized.  Default = pass all, Logging = enabled

/etc/ipnat.conf:
map ep0 192.168.0.0/24 -> 10.160.21.130/32 proxy port ftp ftp/tcp
map ep0 192.168.0.0/24 -> 10.160.21.130/32 portmap tcp/udp 30000:60000
map ep0 192.168.0.0/24 -> 10.160.21.130/32

Any suggestions would be appreciated.

Thanks,
Jared