Module 2: Implementation & Testing: Mistake Clinic

This clinic turns wrong moves into reusable judgment. Use it after each practice page and again before the quiz or checkpoint.

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Module-Specific Mistake Radar

Start with these traps. Replace or extend them with real mistakes from your own work.

Mistake to look for	Where it shows up	Symptom	Repair evidence
Finishing Walking Skeleton Lab with only a final answer	Walking Skeleton Lab	The work has no failed case, trace, test, proof gap, or design stress point.	Add the smallest broken example and show the repair that changes the result.
Finishing Test Strategy Workshop with only a final answer	Test Strategy Workshop	The work has no failed case, trace, test, proof gap, or design stress point.	Add the smallest broken example and show the repair that changes the result.
Finishing Quality Gates Clinic with only a final answer	Quality Gates Clinic	The work has no failed case, trace, test, proof gap, or design stress point.	Add the smallest broken example and show the repair that changes the result.
Finishing Implementation Katas with only a final answer	Implementation Katas	The work has no failed case, trace, test, proof gap, or design stress point.	Add the smallest broken example and show the repair that changes the result.
Treating Walking Skeleton: The Thinnest End-to-End Slice as vocabulary instead of a tool	Walking Skeleton: The Thinnest End-to-End Slice	The explanation names the concept but cannot decide between two cases.	Write one example, one non-example, and the rule that separates them.
Treating Vertical Slices Over Horizontal Layers as vocabulary instead of a tool	Vertical Slices Over Horizontal Layers	The explanation names the concept but cannot decide between two cases.	Write one example, one non-example, and the rule that separates them.

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Practice Mistake Checks

Pull any miss from these checks into your mistake log.

Walking Skeleton Lab

Source: practice/01-walking-skeleton-lab.md

For each statement, identify the error:

1. "The skeleton uses an in-memory stub for the DB because the real DB is not set up yet."
2. "The skeleton is throwaway code; the real version will replace it later."
3. "We can skip deploying the skeleton because it runs locally."
4. "The walking skeleton's endpoint must do something useful for a user."

Test Strategy Workshop

Source: practice/02-test-strategy-workshop.md

For each statement, identify the error:

1. "We have 95% coverage, so the code is well tested."
2. "The service test uses a mocked database, so it is an integration test."
3. "Every feature needs an end-to-end test."
4. "Unit tests should mock every collaborator to keep them isolated."

Quality Gates Clinic

Source: practice/03-quality-gates-clinic.md

For each statement, identify the error:

1. "Warnings from the linter are advisory in CI because engineers know what they are doing."
2. "We hit 95% coverage by deleting the hard-to-test edge cases."
3. "Our contract test passes, so we do not need an integration test for that adapter."
4. "Pre-commit hooks slow down commits, so we skip them locally and let CI catch issues."

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Repair Protocol

For each real mistake:

1. Reproduce the failure on the smallest example, trace, proof, query, command, or design sketch.
2. Name the hidden assumption.
3. Repair the artifact.
4. Save evidence that changed: failing then passing test, corrected proof step, revised diagram, safer command, benchmark, or review note.
5. Add one retrieval card beginning with Check... before... or Do not use... when....

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Mistake Log

Date	Mistake	Symptom	Root cause	Repair evidence	Retrieval card
Starter	Pick one radar row above	Explain how it would fail in this module	Name the assumption	Add a counterexample or corrected artifact	Write the card before closing the page

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Completion Standard

• At least five real mistakes are logged.
• At least two mistakes include a counterexample or failing test.
• At least one mistake connects to an older semester skill.
• At least one correction changes code, a proof, a diagram, a command transcript, a query, or a design decision.