Explicit error messages should be provided for incorrect actions related to the process (as opposed to the interface).
Additional Information:
In some cases, error messages may not be needed in direct manipulation interfaces because results of actions are immediately visible or because some types of errors may be eliminated. However, the design strategy of relying on the ability of users to detect errors from the behavior of the user interface, rather than providing error messages, has some potential problems. Direct manipulation interfaces have their own problems, which may lead to new types of errors. Some of these errors may be difficult to detect if they are legal operations with respect to the user interface but undesirable actions with respect to the task domain (e.g., plant operation).