Scope creep
Work that grows beyond what was agreed, a little at a time, without the fee growing with it.
Scope creep is rarely one big request. It is an extra round of edits, a quick banner, a report in a different format, each small enough to say yes to. Over a quarter it can turn a profitable client into a loss.
The fix is a scope written specifically enough to point to, a record of what's delivered against it, and a habit of naming extra work when it's requested: "happy to, that sits outside the retainer, so it will be AED X or it replaces something else this month".