There has occurred to me, another bit of word-botching which very often crops up on the Internet, and is often rubbished on the Net by "defenders of English". This -- along the same lines as "lose / loose" and "rein / reign" (discussed upthread) -- involves the time-honoured, and correct, expression "To (occasionally 'for') all intents and purposes" -- defined by the Shorter Oxford as "for all practical purposes; in effect". That expression often gets corrupted nowadays, to "for all intensive purposes". This annoys me mildly -- but as with the "lose" and "rein" sagas: I don't follow all the way, the grumpy purists who ascribe this misuse just to clueless semi-literates mindlessly lighting on something which looks and sounds similar, but is actually very different.
I envisage some kind of logic at some level of consciousness at work here -- even if fuzzily. The words essentially signify something which can be categorically declared to be a certain way, save for the very occasional, highly rare exception. I imagine people latching on to the similar-sounding "intensive" here, thinking along lines of "the proposition has been intensively scrutinised; and found to hold good 99.5% of the time". Thus, not total gibberish; but having a meaning not in fact hugely far removed from the original.