In a scenario that occurred with us, the need for the completion of a project was escalated so the Due Date was pushed up.
However, this project had a number of tasks associated with it that still had Due Date set to later than the new Project Due Date. As this was the case, the new Project Due Date could not be saved.
Having the ability to select multiple Tasks and edit their properties (such as Due Date) would be beneficial, not just in this scenario, but others as well. (Such as if tasks need to be assigned in mass to another agent or group.)