Documentation
Comments that explain why, decision records that survive the author, and the failure mode nobody plans for: documentation that has quietly become wrong.
The code already says what it does. A comment earns its place by saying why, what constrains it, and what a reader would otherwise reasonably assume and be wrong about.
API docs, ADRs, runbooks, READMEs, domain rules and migration notes are six different products with six different audiences and six different decay rates.
Documentation is trusted, so wrong documentation is acted on with confidence. That makes a stale document worse than no document — and nothing fails when one goes stale.
Context, decision, alternatives, consequences — written once, never edited, superseded when it changes. The only document that cannot decay, because it describes the past.
Proximity and ownership are the only defences against decay that work without discipline. What belongs in the code, what belongs in the repository, and why the wiki entry is the one that rots.