For example, a project may have two tasks, Task A and Task B. Task A currently starts on October 15 and is scheduled for ...

For example, a project may have two tasks, Task A and Task B. Task A currently starts on October 15 and is scheduled for 1 day. Task B has a Must Start On constraint of October 16, and Task B is also the successor task to Task A with a Finish-to-start dependency. If the duration of Task A changes to 3 days,  Project recalculates Task A's finish date to the end of next day on October 17. However, Task B has a hard constraint that requires the task to start at the beginning of the day on October 16. Therefore, this message displays to alert you that there's a scheduling problem and that now there will be negative total slack in your schedule.