If a unique index suddenly tolerates duplicates, or WHERE email = ... misses a row that a sequential scan finds, the index is sorted by collation rules that no longer match what your OS provides. That happens when the C library's sort order changes — most famously in glibc 2.

Source: [Dev.to](https://dev.to/libme/postgres-let-two-rows-through-a-unique-index-the-glibc-collation-trap-after-an-os-upgrade-3ad6)

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