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Chapter 06 · 05

Where it fails

Be honest about the limits, because this is where unsupervised vibe coding burns people:

  • Deep, novel logic. Subtle algorithms, concurrency, anything where being 95% right means being broken. The model is confident even when wrong.
  • Sprawling cross-file changes. It loses the thread across a large codebase and quietly breaks things it can't see.
  • Security and money. Auth, payments, permissions, anything where a plausible-looking bug has real consequences. Review these like your job depends on it, because it might.
  • Underspecified problems. If you don't know what correct looks like, the AI can't read your mind. Garbage spec in, garbage code out.

The failure mode is almost always the same: code that looks right, runs on the happy path, and is wrong in a way you'd have caught if you'd read it. The fix is the review step. There is no skipping it.

Two questions place any task on this map — is the pattern common, and is the result cheap to check:

                  EASY TO VERIFY      HARD TO VERIFY
                ┌──────────────────┬──────────────────┐
   COMMON       │   PURE UPSIDE    │   go slower,     │
   PATTERN      │   (scaffold,     │   demand tests   │
                │    boilerplate)  │                  │
                ├──────────────────┼──────────────────┤
   RARE /       │   read it, but   │   DANGER ZONE    │
   NOVEL        │   doable         │   (novel logic,  │
                │                  │    auth, money)  │
                └──────────────────┴──────────────────┘

Notice the inverse of the rule above: these are the cases that are rare, subtle, or expensive to verify. The pattern is uncommon, so the model is guessing; or the bug is invisible on the happy path, so a quick check won't catch it. That doesn't mean you avoid AI here — it means you slow down, shrink the steps, demand tests, and read every line like it's hostile.

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