Dan, Does the task inspector tell you anything about the task?
MSP sets the Critical flag based on total slack (default: TS<=0 is Critical.) There are three main causes for what you describe: 1. Traditionally most common: The critical-marked task has an unmet deadline or constraint. 2. Least common: The task’s dates are controlled by assigned resources whose working calendars are not well aligned with the project calendar. 3. New common: The incomplete task has an out-of-sequence successor; i.e. the successor has an actual start that violates the relationship. (I first wrote about this for MSP 2016, but it may have started with 2013.) Good luck, tom