A scope of work fails in week three or it never fails at all. The fights about what was promised, what is extra, and who is paying for which assumption all surface around then. Here is the shape that holds, and the section most agencies still underwrite.
Why SOWs blow up in week three
Not in week one, because the client is excited. Not in week two, because the team is still gathering inputs. Week three is the first time someone on the client side compares the work in flight to the page they signed, and the gap between "what we discussed" and "what we wrote" turns into a meeting.
The fix is almost always in the document, not in the conversation. A SOW that lists deliverables without phases, or phases without assumptions, or an "out of scope" section that says nothing concrete, is a document built to argue with later.
The shape that survives
A SOW that holds reads in roughly this order.
Engagement summary. Two paragraphs. Who is doing what for whom, and what success looks like at the end. Plain language. No buzzwords. If your account lead would not say this sentence out loud, do not write it.