More often than not, certain tasks will take longer than estimated and push the project back. This is sometimes unavoidable as there may be many possible bottle necks and obstacles. These can include external suppliers and internal processes or approvals. This can be combated by building in extra time to the project schedule in these cases. And if they end up not occurring the project becomes ahead of schedule. Though when the delays are much longer and unexpected they may cause overall delays of deliverables and the full project.