The most difficult problem I have solved using Chef was automating the deployment of a distributed application across multiple nodes. This involved creating a custom Chef cookbook that would deploy the application components to the appropriate nodes, configure them, and then configure the network and security settings to ensure the application could communicate across the nodes. The cookbook also had to ensure that the application was running and that the nodes were configured to ensure high availability. This required a lot of complex scripting and logic to ensure that the deployment was successful and that the application was properly configured.