flag | = F = | flaky

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 short-lived 1965 version of the ASCII code to the 1967 version (in draft at the time); this was scheduled for Flag Day (a U.S. holiday), June 14, 1966. See also backward combatability.

flag | = F = | flaky