Last Chinese corner at my Chinese school, my teacher and fellow students started discussing gutter oil and my teacher said something I had never heard before about this oil. Apparently gutter oil, the "common name for old cooking oil illegally collected from restaurant waste bins and gutters and reprocessed to sale at lower prices than new oil," is spicy. Thus it will add spice to any food cooked with it.
My teacher said many Chinese can identify if a restaurant is using gutter oil by tasting spice in dishes that are not supposed to be spicy. This unnatural spiciness is being caused by the use of the gutter oil. Thus, some restaurants might only use gutter oil when cooking dishes that are supposed to be spicy so the customers cannot tell if gutter oil is being used.
This is bad news for me; I order spicy dishes or ask service staff to add spice every single time. I hope my requests aren't being translated by sinister chefs as, "just add gutter oil."