Old bug, new route.

Short summary

When CI tests fail on commits that don't touch the failing code, the bug is usually latent—exposed when the diff routes execution through a previously-cold path. The author calls this 'Old bug, new route.' Named patterns like this change your debugging hypothesis: instead of grepping your diff, grep the producer-consumer contract at the failing call site. Three common cases: asymmetric implementations (async vs sync), cross-platform code (POSIX vs NTFS), and feature-flagged paths.

• •Latent bugs surface when new code routes through previously-cold paths, not from new code introducing the bug
• •Pattern name 'Old bug, new route' is a diagnostic prior that changes which haystack you grep—the diff or the contract
• •Most common in asymmetric implementations (async/sync), cross-platform code, and feature-flagged paths

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