Language-level gotchas, runtime errors, compiler and dependency issues.
Our dosing controllers ran for weeks, then locked themselves out on day 49. The interval check compared now >= lastRun + interval, which breaks when the sum wraps past 2^32 milliseconds. How we proved it with a simulated clock and fixed it with unsigned subtraction.
We launched local pricing in JPY, KRW and VND and a customer in Osaka was charged 398,000 yen for a 3,980 yen course. Our code multiplied every price by 100 before sending it to Stripe. How we found it, refunded it and built a per-currency exponent table with tests.
After we set "type": "module" in package.json, jest.mock stopped replacing our mailer and tests started calling the real SMTP client. Static ESM imports are linked before any mock runs. Here is the mechanism and the fix with unstable_mockModule.
One slow partner API call made our whole FastAPI service stop answering, health checks included, until Kubernetes restarted the pods. A synchronous requests call and a time.sleep inside async def were blocking the event loop.
Customer CSV uploads failed with KeyError 'email' even though the header clearly said email. Excel had written a UTF-8 byte order mark, and other files arrived in Windows-1252. We now detect the encoding explicitly and normalise headers.
Our webhook dispatcher threw ConcurrentModificationException a few times a day and silently skipped deliveries the rest of the time. Two bugs shared one list: removal inside a for-each loop and an admin thread editing it concurrently.
E0502 stopped us from updating parent totals inside a HashMap loop, so someone cloned the whole map to make it compile. That clone ran every ten seconds over 1.2 million entries. Here is why the borrow checker was right and the fixes that kept memory flat.
A practical guide to time handling that survives DST switches: when to store UTC instants, when to keep local time plus an IANA zone, how gaps and overlaps behave, which database types to use, and how to test with fixed clocks.
Our PDF invoices and the amounts charged through Stripe disagreed by a cent on 214 invoices in one month. Floats, toFixed and rounding at every step were to blame. We moved to integer minor units with one explicit rounding point.
A grid trading bot written in TypeScript sent quantities like 0.30000000000000004 and got error -1111. Binary floating point cannot represent most decimal fractions, and serialising numbers to strings exposed it. A decimal library end to end and string serialisation fixed it.
A reminder job sent 380 emails to one customer and none to the rest. The code was correct under Go 1.22 loop semantics, but the module's go.mod still said go 1.20, so every goroutine shared one loop variable. Here is the full trace and fix.
After moving a PHP 7.4 app to PHP 8.3, our logs grew to 9 GB a day of null-to-string deprecation notices and filled a disk. We found the real call sites, fixed them by intent instead of blind casts, and blocked new ones in CI.
Our price streaming API grew from 180 MB to 1.4 GB of heap in a day and was restarted by the orchestrator every night. A listener was attached to a shared EventEmitter on every request and never removed. Heap snapshots proved it; removing listeners on close fixed it.
Event countdowns showed NaN days for iPhone users while Chrome was fine. Our API sent dates like 2024-12-3 9:05:00, which only some engines accept. We switched to strict ISO 8601 with an offset and stopped trusting Date parsing.
Our rebalancing bot computed order sizes as fractions of a portfolio and about one order in five was rejected with Filter failure: LOT_SIZE or PRICE_FILTER. Quantities and prices must sit on each symbol's step and tick grid. Reading exchangeInfo filters and rounding with Decimal fixed it.
Customers started seeing checkout warnings that belonged to other people's orders. The cause was a list used as a default argument, created once per worker process and appended to on every request. Here is how we traced it and made it impossible to reintroduce.