The Bug I Never Wrote: What Testing Failure Taught Me About Solana

100 Days of Solana, Day 100 Where I started I'd built REST APIs for years but had never touched a blockchain, or written a line of Rust. The curiosity how blockchain works, started my curiosity. What I expected I came in with a Web2 instinct: tests exist to prove your code does what it's supposed to do. Write the function, write a test that calls it, watch it pass, move on. A "failing test" was something you fixed, not something you shipped on purpose. What changed my understanding The moment this cracked open was building the capstone: a small Anchor program called proof-of-ship that lets a wallet permanently record, on chain, that it shipped something. The rule is simple — one ship record per wallet, forever. The rule lives entirely in the account's seeds: seeds = [ b"ship" , builder .key () .as_ref ()], bump Each wallet's record lives at one deterministic address. Try to create a second one, and init refuses, because an account already exists there. I wrote two tests. The first proved the happy path: call ship() , fetch the record, confirm the name and builder match. The second test is the one that changed how I think about testing: it ( " only lets each wallet ship once " , async () => { let rejected = false ; try { await program . methods . ship ( " Second try " , " This should never land " ). rpc (); } catch ( _err ) { rejected = true ; } assert . isTrue ( rejected , " second ship should have been rejected " ); }); This test isn't checking for a bug. It's checking that a rule holds. There's no function in my program called preventDuplicateShip() . There's no if statement rejecting the second attempt. The rule "one ship per wallet" isn't enforced by logic I wrote — it's enforced by the Solana runtime itself, because the PDA's address already has data in it. My job wasn't to write the rejection. My job was to prove the rejection actually happens. What I understand now On Web2 systems I controlled the whole stack, so "does it work" mostly meant "does the happy pa