Here are a few common errors in speech that I've noticed (one, particularly, seems to be a localized phenomena at the post-secondary establishment of Sheridan College's Musical Theatre program).

What they say: P.S.
What they think they're saying: "by the way"
What they're actually saying: "the following text is written(even though they're speaking, and not reading) after the author's signature(even though they weren't writing anything)"
The logical fallacy here is that a written form can take place of a verbal one. It's as logically confusing as requesting an e-mail with "verbal instructions" when perhaps they mean "explicit," or "detailed," or "written," ...
How unfortunate for the author that the definition and meaning of "verbal" does not include "not visual".

What they say: per se
What they think they're saying: "[not] necessarily" (adverbial)
What they're actually saying: "in and of itself"
Here, in an adolescent abuse of attempted academic advancement, this latin phrase has been beaten and battered into serving the function in the place of a perfectly legitimate English phrase. The statement "he didn't die, per se," is now more commonly interpreted to mean "he didn't die, exactly" or "he didn't die, necessarily", when really, they're saying "he didn't die, in or by himself," which really makes no logical sense.
The statement, "that painting is beautiful, per se" actually means that the painting refered is beautiful on its own; it's intrinsically beautiful, rather than some clever arrangement of its lighting or framing, or location.

What they say: i.e.
What they think they're saying: "e.g.; for example"
What they're actually saying: "that is; specifically"
Often, the two are reversed, in which one will incorrectly say "e.g." when they in fact mean "i.e."
The difference between the two is actually quite easy to discern. The latin phrase "id est" (i.e.) means literally, "that is", and is used to specify something specifically after speaking about it generally. "E.g.," on the other hand, stands for "exempli gratia" which quite literally means "for example" or "for the sake of an example".
Thus, if I were talking about the gravitational pull of varying stars, it would be correct for me to say, "Stars with the greatest gravitational pull, i.e. black holes, ..." It would be inappropriate to say "e.g." in this case because only one, the black hole (or Schwartzchild Singularity, if you want to be historical), has the greatest gravitational pull.
And, if I wanted to illustrate what I meant, when talking about something highly abstract, it would also be appropriate for me to say, "derivational adverbs may be formed when the schwa vowel of the root word is maintained, and the morpheme -ly is appended to the end. E.g. lively.
A better example of displaying what I mean by the abuse of the two, I write the following two sentence fragments:
Those who gave birth to me, i.e. my parents, ...
Those who gave birth to me, e.g. my parents, ...
The first means that "my parents" is being specified and identified as the previously ambiguous form of "those who gave birth to me", whereas in the second sentence means something closer to, "those who gave birth to me, among whom were my parents, ..." which really makes no sense.

What actually impresses me is not so much the low calibre with which we seem to satisfy public education, so much as the ingenuity for individuals to corrupt and twist established words and meanings into illogical and unnatural concepts. It really gives rise to the despair.com quote, "none of us is as dumb as all of us!"