The failure mode is absence, not interference
Projects rarely fail because a client was too involved. They fail because decisions waited weeks, feedback arrived after the thing was built, and nobody on the client side had authority to choose. Engagement is the variable that most predicts the outcome.
Name one decision-maker
One person who can settle a question the same day, with the authority to disappoint a colleague. Committee feedback arrives contradictory, and reconciling it becomes work you are paying for at agency rates.
The most expensive line in any build is the week spent waiting for a decision nobody was empowered to make.
Review in the medium it ships in
Feedback on a static image of a page is guesswork about how it behaves. Review working software on real devices with real content — the gap between an approved mockup and a built page is where most late-stage change requests originate.
Give feedback as problems, not solutions
“Make the logo bigger” is a solution to an unstated problem. “Nobody can tell whose site this is” lets the people you hired solve it properly, and usually produces something better than the instruction would have.
Supply content early or accept the consequence
Content is the most common cause of delay on otherwise healthy projects, and design built around placeholder text breaks when real copy arrives. Agreeing who writes what, by when, belongs in the spec that gets quoted.
Agree what handover means
Code ownership, credentials on your accounts, documentation, and who maintains it afterwards. Deciding this at the end produces an awkward conversation; deciding it at the start is a paragraph — and it is what stops the build becoming software nobody owns.