A plan that fails in year two normally failed in the week it was written. The execution gets the blame because that is when the problem becomes visible.
The unexamined assumption
Every plan rests on a handful of load-bearing beliefs: how many customers exist, what they will pay, how long they take to pay, how much it costs to reach them. Most plans state the conclusions and leave the beliefs implicit.
Write them down separately, each as a sentence with a number attached. The list is usually shorter than people expect — five or six items carry the whole plan. Then ask, for each, how you know. “A supplier told me” and “we measured it over three months” are very different foundations, and the difference should be visible.
Time is the assumption that breaks first
Founders who are careful about price are frequently careless about time. Permits take longer. Hiring takes longer. The first customer takes considerably longer, and the second one does not arrive on the schedule the first suggested.
A plan that survives contact has slack in the timeline and enough funding to absorb it. If the model only works when everything happens on schedule, it is not a plan, it is a best case.
Write down what would prove you wrong
The most useful page in a business plan is the one nobody includes: what you would expect to see if the core assumption were false, and by when.
Decide in advance that if you have not signed three customers by month four, the pricing needs revisiting. Committing to that while you are calm is what makes it possible to act on it later, when you are not.
A plan is not a prediction. It is a statement of what you believe, specific enough that reality can correct it early — while correcting it is still cheap.