Write regression tests for all sorts of bugs you find.
The first step if you find a bug through production use should be to write a failing test for it, even if the software isn't being developed through TTD.
Write UI tests.
Use fuzzers.
Use mutation testing.
Use TypeScript/mypy
Use linters.
Use formatters. Yes, formatters. Having your code consistently formatted will instantly reveal when you mistyped something parseable but different from what you meant.
And it will minimize git diffs, making them easier to review.
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Some stuff I'd like to add: