Many models rely on the assumption of nominal price stickiness. But the different definitions of frictions can greatly alter their macroeconomic implications. In this column, price stickiness is modelled as the result of errors due to costly decision-making. Errors in the prices firms set help explain micro ‘puzzles’ relating to the sizes of price changes, the behaviour of adjustment hazards, and the variability of prices and costs. Errors in adjustment timing increase the real effects of monetary shocks, by reducing the ‘selection effect’.