Flag Day: N. A Software Change That Is Neither Forward- Nor Backward-compatible, And Which Is Costly To Make And Costly To Reverse.

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:flag day: n. A software change that is neither forward- nor
backward-compatible, and which is costly to make and costly to
reverse. "Can we install that without causing a flag day for all
users?" This term has nothing to do with the use of the word
{flag} to mean a variable that has two values. It came into use
when a massive change was made to the {{Multics}} timesharing
system to convert from the old ASCII code to the new one; this was
scheduled for Flag Day (a U.S. holiday), June 14, 1966. See also
{backward combatability}.
-- The AI Hackers Dictionary