flag day
A software change that is neither forward- nor
backward-compatible, and which is costly to make and costly to
reverse. E.g. "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} {time-sharing} system
to convert from the old {ASCII} code to the new one; this was
scheduled for Flag Day (a US holiday), June 14, 1966.
See also {backward combatability}, {lock-in}.
[{Jargon File}]
(1998-01-15)