My poking around in the GECK has turned up lots of interesting tid bits. This one really bites though.

I found a schematic named Mantis Scythe that was not finished out as being a playable /craftable weapon.

Fallout wiki even has some info on it.
http://fallout.wikia.com/wiki/Mantis_Scythe

WHAT TH HELL!?!?!!?

On second thought. I guess that not being able to craft it isn't such a big deal, as you can still buy
it from time to time.