Slitherine

I am playing as the Entente and the "Arab Revolt" just began in the Hijaz.

The production menu for Arabia is porked. When I click on it I get Serbian units. When I click on them I get superimposed Serbian units with Armored Cars.

I saw this happen once, just like you describe. I tried to purchase a unit and the game crashed to desktop.

After reloading the autosave, Arabia didn't appear in the Production window anymore (which, I suppose, is the correct behavior).