Some bugs announce themselves with a stack trace. Others just make your code quietly stop meaning anything. This is about the second kind — a dependency resolution issue in a TypeScript monorepo that didn't throw, didn't fail CI in an obvious way, and took real digging to root-cause, because the symptom looked nothing like the cause.
The symptom
We're a Next.js + TypeScript monorepo using npm workspaces, on top of Supabase/Postgres with row-level-security policies. One afternoon, a teammate opened a PR, and the type checker flagged what looked like an unrelated function as having a parameter of type never. Not unknown, not any — never. That's the type TypeScript uses to say "this code path is unreachable" or "no value can satisfy this type."
The function in question was very much reachable. It was called constantly, in production, successfully. TypeScript was simply wrong about it — or rather, TypeScript was being correctly told the wrong thing.
Why never is the worst possible symptom
