That these mistakes are oft repeated doesn't make them any less wrong.
Absolutely it does make them less wrong. Are you going to tell me that using phrases such as "to curry favor" and "moot point" are incorrect because a long time ago someone misheard someone else say "to curry Favel" and "mute point"? (see: http://blog.oxforddictionaries...)
Everyone should agree that languages evolve over time. For some reason, however, some people get really indignant when they observe the actual mechanisms by which languages evolve up-close.