I was just surfing around, looking for a decent grammar guide for a friend, when I ran across this site.  The fella there has an impressively long list of common English errors, but what I found most interesting is his list of non-errors.  Non-errors are caused by pedants who are so eager to show off their superior knowledge of the language that they will go too far and actually show their ignorance.

I chuckle inside when I hear someone saying "I feel well" or "between you and I."  I don't fault them for trying to speak proper English; heck, I've often laughed at myself for saying such things, and I immediately correct myself (often under my breath) to remind myself.

You'd be surprised at the number of bad habits I've had to break, things like "coulda went," "less calories," and one that I still have trouble with: "I'm gonna go lay down for a bit."  What can I say?  I was raised in Arkansas and Oklahoma, neither of which are particularly known for proper English. :)

For the record, the proper way to say the above things are:
1. "I feel well" should be "I feel good," unless of course you are talking about your health and not your state of mind.  If you are thinking "I feel good" (like the song), then you should say that and not overcorrect yourself.
2. "Between you and I" should be "between you and me."  This is a case where folks have been told that saying "you and me" is incorrect, usually in the context of using it as the subject (nominative case) in a sentence such as "you and me are going to dinner."  In that case, you should use "you and I" because "I" is the nominative case for the singular, first-person personal pronoun; however, nouns that follow prepositions (which is what "between" is) should be in the accusative or dative case, and that case for the aforementioned personal pronoun is "me."
3. "Coulda went" should be "coulda gone" if you're intending to be colloquial.  Of course, the more proper way would be "could've gone."
4. "Less calories" should be "fewer calories."  This is a toughie, at least for us Americans.  "Less" should be used when referring to a single thing, such as "less water" or "less sand."  "Fewer" should be used when talking about multiple things, such as "fewer items" or "fewer calories."  Even Wal-Mart gets this one wrong, or at least they intentionally use it because it is more common to be incorrect.  Let me know if you find a store whose express lane says "10 or fewer items" rather than "10 or less items."
5. The last one that still causes me trouble is "lay" versus "lie."  The difference is in whether or not the verb takes an object, that is, whether or not it is transitive or intransitive.  If you are putting something down, you would "lay" it down; however, if you are describing what something is doing, you would say it is "lying" down.  So I should say "I'm gonna go lie down for a bit" unless I'm going to go take something and lay it down for a while. 

Anyways, for the time being, these are still shibboleths, but I wouldn't be surprised if in a few decades these become the new non-errors because common usage has made them correct. 

The main thing to keep in mind is that there are plenty of folks who use the way you speak or write as a means to determine your level of education or, worse, intellect.  It's not so much that these little errors really matter in getting your point across (which is, after all, the point of language) but that you don't want to be discounted a priori for what is considered by many others to be incorrect.