What are some situations that might cause a project baseline to legitimately be reset? As a project manager, how do you address the changes?

 Pinto (2013) discussed the project baseline as a point to establish ongoing status of the project budget in controlling the cost of a project. In addition to cost, project schedules are monitored in the same way. In the business industry, setting a baseline becomes important in measuring overall project success. What are some situations that might cause a project baseline to legitimately be reset? As a project manager, how do you address the changes? What are some problems that can be encountered with milestones?