We usually think of contractions, such as “it’s” or “can’t,” as directly interchangeable with the full versions, such as “it is” or “cannot.” You can just replace “it’s” with “it is,” or vice versa. Both versions are commonly used, both have the same meaning, and the difference is just in degree of formality.
But in one common context, it turns out that you don’t have that option: You can’t just replace the contraction with its spelled-out version. The contraction is standard, but the direct conversion to the full version is so very rare that I would label it as nonstandard. What is that context?
Two weeks ago, I don’t think I could have answered that question. The rule, I think, is so ingrained and so rarely violated that we don’t pay attention to it. It took a conversation with my 6-year-old, in which he (perfectly logically but jarringly) uncontracted the contraction to make me notice this.
Technical notes: When I say “nonstandard,” I simply mean that the usage is almost never seen or heard, at least in American English. I’m not making a prescriptivist claim about what should be done; spelling out the contraction would be quite logical — it just isn’t done.
UPDATE: “O’clock” is not what I had in mind, though I suppose it would qualify. I’m speaking here of a particular grammatical context in which a wide range of contractions are standard, but their spelled-out equivalents are nonstandard.