War story: the hardest bug I ever debugged

Original Article ↗ Hacker News Discussion ↗

Article bug & reactions

  • Many liked the writeup but questioned calling a 2‑day hunt “hardest ever,” arguing truly brutal bugs take weeks or months and are barely reproducible.
  • Others countered that difficulty isn’t just elapsed time: tracking a nondeterministic crash into a JS engine optimization tier and proving Math.abs was wrong is inherently gnarly.
  • Several noted how exhausting “brute-force grind” debugging can be, especially under a culture that normalizes grinding on top crashes.

Testing, compilers, and optimization tiers

  • Commenters critiqued V8’s testing: if an optimized tier had a separate implementation of Math.abs, tests should have exercised that path and enforced coverage.
  • There was discussion of how “rarely used super-optimized modes” are risky if not regularly and systematically tested, and how combinatorial config spaces make full coverage infeasible.
  • Suggestions included stochastic/continuous testing over random (test, config) pairs and “force this optimization mode” flags to run suites under each tier.

Heisenbugs and rare, environment-driven failures

  • Many shared “hardest bug” stories: month/years‑to‑repro issues, PLCs, network appliances, shady NIC drivers, miswired hardware, and compiler/driver bugs.
  • A common theme: Heisenbugs that vanish under instrumentation, or only appear in production hardware, or when specific timing, thermal, or load conditions are met.
  • Hardware examples emphasized how probing or logging can change behavior; cosmic‑ray/bit‑flip explanations came up for truly one‑off failures.

Security and JIT implications

  • One thread explained how a miscompiled Math.abs can be exploitable: JITs remove bounds checks based on assumptions like “abs is non‑negative,” so wrong code can yield out‑of‑bounds memory access and array length corruption.

QA, tooling, and organizational factors

  • Several comments stressed the value of dedicated QA and exploratory “off happy path” testing; engineers tend to validate only the designed flow.
  • Vendor and organizational issues (poor docs, lying or clueless support, incompatible driver/OS changes) were often what made bugs truly hard.
  • A meta-thread noted how often multiple teams independently chase the same deep bug, or how long‑fixed upstream bugs still consume downstream engineers.