Things I changed my opinion on based in practice - if you are committing a change, a test change should be included.
If you're modifying something, then either
a) it was tested before, and something must have started failing, or b) you're modifying something that wasn't tested before, so a new test case should be added.
Obviously one should apply common sense, but it's useful even for small/one-off projects...
Example where this is being done rigorously - SQLite, e.g. https://www.sqlite.org/src/info/3dc9fc2f2de8696b
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