Project plans should try to plan for some unknowns. For large projects they include known risks, vacation time, maybe even the loss of a key person or a parallel business crisis. For smaller projects it’s just x percent of slack time that can be used as needed. You do what you can up front, adjust course as needed. Standard stuff in our profession, at least in theory.
My stance when it comes to projects it is “win early”. That means push hard today, try to get to and conquer the unknowns as soon as possible. It’s not the easiest strategy to implement – it can lead to pressure and urgency just for the sake of it. The goal is to not get complacent, not to get caught up in committee meetings, to try to make decisions now, or at least tomorrow, to deal with risks sooner rather than later.