No project plan ever goes to plan. You make them up based on assumptions. How can you plan something, sometimes months in advance based on assumptions and expect to be right. Plan ahead for this and expect something to go wrong. Plan for things to take longer than you think they should. Plan for things you can’t even think of. Plan extra time into the plan so you can adapt when you need to. It’s easier to say you planned ahead for unplanned things than having to say the project is no longer viable because you ran out of time.