Linkletter v. Walker
The term "retroactive" means something that refers to facts or events in the past, and a retroactive law is one that applies to past events. The Omnibus Budget Reconciliation Act of 1993 (OBRA), for instance, retroactively raised 1993 taxes on a number of individuals in the newly created 36 percent and 39.6 percent tax brackets.
Is this fair? The retroactivity provisions in OBRA were legal if not necessarily "fair", but generally, common law has tended to hold retroactive laws in disfavor. As for criminal law, the Constitution renders retroactivity illegal through its prohibition of ex post facto laws.
There would seem to be little to justify retroactivity, at least from the perspective of the individual and not the government. It is conceivable, that a retroactive tax law could benefit an individual by lowering taxes in a certain category for a certain year. Retroactive application of a civil-rights law could help members of a minority group subjected to past discrimination.
In any case, judicial decisions are, unlike sentences in criminal law, retroactive. This retroactivity is almost always at the legal rather than the practical level, however, because actual attempts to change a past condition would most likely lead to violation of someone's rights.