Yesterday, I wrote about eggcorns, mondegreens and malapropism. Phenomenons that change the language due to a simple mishearing or by an improper usage.
Such an eggcorn is “for all intensive purposes” and “for all intents and purposes“; the later the correct one, the former the eggcorn. “For all intensive purposes” seems to have first appeared in the 1950s, while a version of the standard idiom, “to all intents, constructions, and purposes,” is cited in the Oxford English Dictionary as early as 1546. Historically, the dominant form of the idiom was “to all intents and purposes“, but over the past several decades that has gradually waned as the “for” form has increased in popularity.