We can think about this danger by considering a supply shock, that causes the relative price of energy to increase sharply, as in fact has happened over the past three years. Let us suppose that we start out with both headline and core inflation at acceptably low levels.
First, let's consider a supply shock that temporarily raises the price of energy by a large amount. In this case headline inflation will rise well above its underlying trend as the price of energy rises but will soon fall well below its underlying trend as the price of energy falls back to its initial level.