Gold plating
Adding features, polish, or engineering refinement beyond what the requirements asked for, work that consumes effort without delivering agreed value. Gold-plating usually comes from good intentions, an engineer 'improving' a component, but it inflates scope invisibly and is the internal sibling of scope creep, originating inside the team rather than from stakeholders.
Gold plating is effort spent past the point of value: polishing, generalising, or adding capability nobody asked for. It feels like craftsmanship but it's really scope the customer didn't request and won't pay for, and it crowds out work that would. Acceptance criteria are the antidote, they define "done" precisely enough that "better than done" becomes a visible, deliberate choice rather than a default an engineer quietly drifts into.