I'll point out straight away that I was testing a manual build of a fork, so the issue might not be present in the core mod, but from what I gather it probably is.

When starting my server, I got a cpw.mods.fml.common.LoaderException: java.lang.NoClassDefFoundError: net/minecraft/client/gui/Gui at com.hbm.items.weapon.sedna.factory.GunFactory.init(GunFactory.java:23). I think I've tracked the issue down to the Crosshair enum. The problem seems to be that the enum is declared in RenderScreenOverlay.java, which imports a lot of machinery related to client-side rendering, not available in a server environment. I guess just moving the enums out of the rendering-related code should solve the problem for good.