My favorite class of bug is the one the database makes impossible. The treatment estimator rewrite freezes every price at the moment it's captured, with write-once columns enforced by triggers and full revision snapshots on every status change, because a patient quoted $38,000 in March deserves the same number in September, and a policy memo is weaker than a constraint.
I hold solo projects to team standards, because when you're the only engineer there is no one else to catch it: strict typing across the board, four-tier test pyramids with golden-file fixtures, idempotency at every boundary so replays never duplicate, CI/CD with keyless auth, and infrastructure that rebuilds from a clean checkout with one command.
The discipline pays off in unglamorous ways. The dashboard ships with a second, independent implementation of every metric purely as an audit oracle. The accounting platform's tests verify outputs byte for byte. The NPC pipeline's 1,156 tests run against in-memory fakes, so no test ever touches real patient data. Boring on purpose, and durable because of it.
