# M06-L04 exercise — One discriminating change

Starting checkpoint: CP06. Suggested time: 30–40 minutes.

Capture the original invalid-fixture command, output, and exit. Write two plausible failure locations and a predicted observation that would distinguish them. Inspect the documented grammar and relevant adapter code before changing a file.

Create a separate copied fixture and correct only `28C` to `28`. Run it, then rerun the unchanged original. Record what both observations establish. Explain why changing the parser to ignore suffixes or replacing the value with zero would answer a different question.

Submit a short fault report containing starting state, reproducer, expectations, hypotheses, chosen test, actual results, correction, and limits. If an assistant drafts the report, review every claim against the preserved evidence. A successful parse does not establish sensor operation, calibration, or physical timing.