> Commands should be left enabled, then an error message should be displayed when the command is clicked and the command is unavailable, and in this case explai

> Commands should be left enabled, then an error message should be displayed when the command is clicked and the command is unavailable, and in this case explain why it is presently unavailable. An exception to this: premium (for-pay) features. There is a certain To-Do app that does this. Premium commands are visually indistinguishable from freemium commands, so they get clicked a lot and they interrupt the entire experience with nag dialogs. A serious UI error IMO.