One particular project I had worked on at a drug discovery & development laboratory ran into the issue of missing a milestone. In this situation, the Gantt chart had planned for 4 researchers to work on a complete the initial R&D along with half of the synthesis in 3 weeks. However, 1.5 weeks in, one of the student researchers working in the team contracted mono and was unable to complete his part of the project. This caused us to be behind on the synthesis of the particular drug at the 3 week milestone. To remedy this issue, the lead professor decided to purchase a large segment of the final drug (we had normally made all our drug compounds from scratch) which got us back on track according to the Gantt chart but costed the chemistry department ‘unplanned’ money.