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This essay, Mirror test, refers to running tests, such as for template operations, based on incorrect, or invalid, test data, which can give the appearance that there are defects, or perhaps nothing might appear wrong, when in fact, the real errors are in the test data instead. The essay title "Mirror test" relates to the main mirror for the Hubble Space Telescope, which was carefully polished to incorrect mirror specifications and passed tests against the incorrect test data. When running tests, beware any tests run against live articles, which might have errors in the markup language, rather than assumed to be "perfect" articles to be used for comparison tests.