When shipping code or fixing bugs, my biggest concern is enforcing invariants across the entire application so logic does not drift or behaviour change unexpectedly.
Backend coverage alone is not enough because users can take valid actions in unexpected sequences. Interfaces enforce structure, but not consistent business behaviour across implementations. Shared domain rules, contract tests, state transitions, and factories provide important code-level safety nets, but automated end-to-end tests offer the strongest protection by validating that the application still behaves correctly as a complete system.