I cook a lot and cannot think of any totally non-scaling ingredient, although some things go out of proportion a bit; e.g., the relative volume of water used to cook rice goes down as the recipe grows.
I feel like this is pretty well known; not that recipes can't be scaled, but that you can't reliably do it by multiplying the ingredients.
I just flipped through Ferran Adria's "The Family Meal", which is a record of every house meal served at El Bulli, each of which is scaled to 2, 6, 20, and 40 people. Sure enough, almost none of them scale every ingredient linearly (Caesar salad and hamburgers did, because you just make more dressing or more patties for more people).
In particular: oil, butter, lemon juice, onions, and intensely flavored seasoning ingredients (herbs in particular) often scaled by 2x when the servings scaled by 3x. In some places, an ingredient scaled by more than 3x to make 3x as many servings. Salt went in both directions.
