Antitrust authorities often consider parallelism of firms' strategies and market share stability as clues of illegal collusion in the form of concerted practices. In this paper I show that this inference may be contrary to some theoretical results. I develop a model of price competition with differentiated products in which demand and costs vary over time. If the market is perturbed by shocks on the supply or on the demand side which are common to all firms, then price parallelism will occur both in a competitive and in a collusive equilibrium and cannot signal which one occurred. If shocks are firm specific, price parallelism connotes a competitive market. The paper also shows that the competitive equilibrium is characterized by a higher market share stability than a collusive equilibrium.