It won't cause any errors, it just won't work.
My attempt to do so was when I was trying to design boarding shuttles, ala the Thebans from 'Insurrection' by David Weber and the Starfire game.
60 kt fighters that had to mount 50% as boarding crew. Unfortunately, the computer didn't make any such requirement when I went to design one in game, and I ended up with a 60 kT fighter, a bit too large and powerful for a fighter
Oh well.
Derek