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Postmortem: How an LLM Hallucination in GitHub Copilot X Cost Us 3 Days of Debugging

DEV Community·ANKUSH CHOUDHARY JOHAL·28 days ago
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We burned 72 engineering hours—three full days of senior backend developer time—because a GitHub Copilot X LLM hallucination suggested a syntactically valid, semantically broken Redis cache invalidation pattern that passed 142 local unit tests but spiked our production p99 latency to 11.2 seconds, dropped 14% of user requests, and triggered 3 SEV-2 incidents before we traced the root cause. 📡 Hacker News Top Stories Right Now About 10% of AMC movie showings sell zero tickets. This site finds them (86 points) What I'm Hearing About Cognitive Debt (So Far) (161 points) Bun is being ported from Zig to Rust (365 points) Train Your Own LLM from Scratch (56 points) CVE-2026-31431: Copy Fail vs. rootless containers (59 points) Key Insights LLM-generated code with 100% unit test coverage introduced a 400% p99 latency regression in production Redis workloads. GitHub Copilot X (v2.1.7) hallucinated a non-existent Redis Lua script return value contract for the redis.call('del') command.…

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