Do unexpected events experienced by one line of business adversely affect other lines of business in diversified firms? We use fine-grained data on the film industry in the United States to show that such contagion frequently occurs when a distributor opens a film in theaters and concurrently releases an older title to home video: Being exposed to a competitive threat-a period of unexpected volatility-in the theatrical market at the time of a film opening leads the distributor to suffer a loss in sales on the concurrent home video release. Further analysis revealed that managers responded to these competitive threats by intensifying the advertising and promotion of their films in theaters, suggesting that they diverted resources and attention away from home video. Our results therefore suggest that the effects of unexpected events do spread across lines of businesses within firms and consequently that resource constraints may limit the ability of firms to engage effectively in multiple markets.