Next up on the list of ongoing changes to the English language is something that has become so uncommon that many native English speakers don't even know about it - an indication that the loss of it is at a very late stage, making it almost certain that it will eventually completely disappear from English. Consequently, I'll have to take a bit to explain it.
Let's suppose, for the sake of explaining this, that English had the distinction between subjective and objective case for nouns and adjectives, as it still does with pronouns. For a simple sentence, such as "The dog has a bone", it's clear that "the dog" is in the subjective case and "a bone" is in the objective case, as they are the subject and direct object, respectively. Adjectives are also pretty intuitive. In "The stupid dog has a bone", you'd probably guess (correctly) that "stupid" would also be in the subjective case; whereas in "The dog has a big bone", "big" would be in the objective case.
Let's make it slightly harder. In "Biggin's, my dog, has a bone", what do you suppose the case of "dog" is ("my" is clearly in the genitive case, and "Biggin's" and "a bone" are also obvious)? We might imagine a language which features an elaborative case - a case used for stating information elaborating on a previous noun; however, I don't know of any such languages, and Indo-European languages certainly are not. In Indo-European languages, elaborations such as this, similar to adjectives, agree in with the thing they are elaborating. So here, it would be the subjective case. This becomes even more intuitive when you consider that "My dog Biggin's has a bone" means exactly the same thing.
Given that information, you should have no trouble figuring out the cases of nouns and adjectives in "Kaity, my fat cat, likes Biggin's". But suppose we have "Kaity is a fat cat"; now what? "fat cat" is acting as the direct object of the verb, so you might think it would be in the objective case. While this is very common these days, it's wrong. "fat cat" is called a predicative - a noun or adjective phrase in a descriptive sentence that elaborates the subject - and is a special case of the rule stated in the last paragraph; in Indo-European languages, predicatives agree with the subject - the thing they are elaborating.
Honestly, even I don't know how to use predicatives correctly 100% of the time (and I actually do use them correctly even less, as they tend to sound archaic to modern English speakers; which sounds better - "That would be I" or "That would be me"?). Specifically, I don't know exactly how many verbs they apply to. I know they apply to any sentence of the form "[something] is [some noun, adjective, or pronoun]". Same with some verbs of perception, such as "appear". I believe they also apply to some other cases, where there is a comparison of two things with regard to a particular adjective, but I don't know the rules for that very well.
Anyway, the reason this is being lost should be obvious: English has not had a distinction between subjective and objective case for nouns and adjectives for a good 1000 years. This makes it much less used now than it was then, as it's rare for there to be an elaborating pronoun in the predicate. And as with all things related to language, the less frequently something is used, the more likely it is to mutate or be lost (for example notice has the verb "be" has retained far more peculiar conjugations than any other verb in the English language; that is because it is the most common verb in the language).
Occasionally, you'll see someone use the subjective case for one of the verb objects or a prepositional object, even though the noun should be in the objective case. This is often a case of hyper-correction. A person who is not familiar with predicatives may hear them used, perhaps at some high society gathering, and attempt to imitate them to sound more educated than they actually are. Naturally, as they don't understand the rules for predicatives, they end up misapplying them, with incorrect and awkward-sounding results.