n. A known software flaw that never gets fixed, particularly one that exposes a security hole.
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To make matters worse, many industrial control system vendors are
not committed to fixing the security holes that exist in their deployed products, especially legacy products, resulting in what are called "forever-day bugs".
—“Special Track: Cyber and Smart Grid Information Security” (PDF), The 2013 International Conference on Security and Management, February 21, 2013
Forever day is a play on "zero day," a phrase used to classify vulnerabilities that come under attack before the responsible manufacturer has issued a patch. Also called iDays, or "infinite days" by some researchers, forever days refer to bugs that never get fixed—even when they're acknowledged by the company that developed the software.
2012 (earliest)
Zero-day as it applies to software exploits is different from zero-day as it applies to non-software exploit based malware. If by "bug" he means 'software flaw' then such a 'bug' can exist for a long time without any vulnerability or exploit ever existing because of it. So 'zero-day' becomes closer to 'forever-day' in such a case.
—FromTheRafters, “Do we really need to keep using” (comment), PC Review, February 17, 2012