The worst mistake in this trade takes one second. A post intended for one client appearing on another's account is public, screenshotted and remembered, so separation between accounts has to be structural rather than careful.
Approval deadlines run ahead of publication. A post scheduled for Monday morning needs approving on Friday, and a chase that starts on Monday is a chase that has already failed.
Response time is a promise that decays quietly. Without measuring how long messages actually wait, an agency believes it replies quickly right up until a client counts for themselves.
Reports should resist the easy numbers. Followers and impressions rise on their own, so a report built on replies, saved posts and enquiries received is harder to produce and much harder to argue with.