A bug in the sun3 netboot has been identified and fixed.

The function netif_get() in stand/libsa/netif_sun.c can
overwrite the passed receive buffer (usually on the stack)
if any packet longer than the expected one arrives.  This
error may cause netboot to crash or fail unpredictably.

A new version of netboot (with the error corrected) is
available on the FTP server.  If your system is behaving
unpredictably during netboot, please try the new version.

How get it:

ftp ftp.netbsd.org
cd /pub/NetBSD/arch
binary
get netboot
bye