I was making a new helm that was slightly lighter and more protective than the vault security one.
Basically, I just opened the helmet entry in the GECK, renamed it, then saved it as the new name.
I didnt touch the .nif at all.
Simple as you can get.

But when I picked it up and donned it in-game, here is what I saw!
http://i237.photobucket.com/albums/ff292/VonGrantoven/Neckprotector.jpg

Why did this happen?
How can I fix it or avoid in future?

It is always a good idea to search the forum

http://www.gamesas.com/?showtopic=1001497&st=0&p=14950252&

Thanks Ashara.
I did run a search, but frankly I never seem to get any decent results when I use the search engine on this site. In fact I just ran "helmet side neck" and it never even came up with this thread.

In any case, the "update facegen model availability" did the trick.
Thanks for the help!

Great!