I Wrote 238 Tests Against My Own Auth Package and Found 4 Real Bugs

I'd already done a lot right by the time I started writing tests for Beaver-Auth . Every module had gone through multiple rounds of deliberate review. Enumeration protection, hashed tokens, refresh rotation, TOTP replay defense — the design was solid, and I knew it was solid, because I'd thought hard about every piece of it. Then I wrote 238 tests against the actual code, and found 8 real bugs. Some of them were the kind that would have silently broken production on day one. This post isn't about the bugs specifically — it's about the gap between "I reviewed this carefully" and "this is shippable," and why that gap is bigger than most of us assume, even when the reviewing was genuinely careful. "Passing tests" and "shippable" are different claims Here's the trap I nearly walked into: I'd built a solid test suite covering the core auth flows — registration, login, verification — and every test passed. It felt done. But passing tests only tell you the code does what the tests expect. If the tests were written from the same mental model as the code, they'll happily confirm a bug is correct behavior, because both the code and the test agree on the same wrong assumption. The fix wasn't "write more tests." It was testing against the real, integrated system — not a hand-built mock of my own logic, and not testing modules in isolation from what actually calls them. A few of the bugs below only surfaced because a test exercised the real dependency chain instead of assuming it worked. Bug 1: TypeScript let an argument-shift bug compile clean This is the one that scared me most. Beaver-Auth dispatches background work (like sending a verification email) through a TaskDispatcher interface: interface TaskDispatcher { dispatch ( taskName : string , payload : unknown , handler : () => Promise < void > , onFailure ?: ( error : unknown ) => Promise < void > | void , ): Promise < void > } The default implementation had drifted to a different signature — missing the payload parameter e