Grandfather Clause

A clause in a rule or law which exempts existing persons/things/etc. from the effects of the rule. Derives from clauses in various voting acts passed in the post-CivilWar? American South, intended to prevent African-Americans from voting (in defiance of the 15th Amendment, which gave blacks the right to vote). The laws erected difficult-to-overcome barriers to voting which were purportedly color-neutral; but contained the exemption that if your grandfather was eligible to vote, then so could you. As the grandfathers of most blacks were slaves, blacks didn't qualify for this exemption, but most whites did.

Eventually, most such laws were thrown out as unconstitutional, but not until significant damage was done.

How is this related to software? OffTopic?

It is used to describe a number of WikiSocialNorms, and, more notably, their exceptions.