with TXCSUM on, the em failes to send a UDP packet to port (short)-1 (0xffff)
workaround: ifconfig em0 -txcsum
How-To-Repeat: nc -u some-host 65535
and watch via wireshark
Over to maintainer(s).
I've fired off this exact test on my 82574L based host (em) and it works for me.