"Must of " vs "must have"

It used to be that people said "I had rather do this" (contracted to "I'd rather").

Nowadays, this sounds incredibly old-fashioned. People say "I would rather" (contracted to "I'd rather").

Of course, they're identical when they're contracted, which is probably responsible for the shift.

A slightly more tricky example of wrong grammar becoming acceptable is usage of who vs. whom. Whom is the object case of who, but nowadays who is also fully acceptable for this purpose. As a consequence, some people are using whom in novel ways. They are already so numerous that it can't be called wrong any more.

In traditional grammar, who(m) when used as a relative pronoun takes its case from its function within the relative clause. (This is also how such things work in all other Germanic languages.)

• I gave it to him who needs it.
• The man whom we saw was in a hurry.

In the course of a general simplification trend of English grammar, who became acceptable instead of whom:

• I gave it to him who needs it.
• The man who we saw was in a hurry.

Then some people used whom as a marker of formal speech, even in situations where it doesn't fit. (Presumably because they didn't understand the rule any more.)

• I gave it to him whom needs it.
• The man whom we saw was in a hurry.

And nowadays there are even people who use whom as the form of the relative pronoun if and only if it refers to something that has the object case in the main clause:

• I gave it to him whom needs it.
• The man who we saw was in a hurry.

The issue prompted James Thurber to his hilarious 1929 satirical advice on proper distinction of "Who and Whom", and the confusion didn't get less since then.

You could look to to old meanings of words and check the time period it changed.

An example would be prove, like "The exception that proves the rule." as prove today means to demonstrate as true by evidence.

Today that sentence means "The exception provides evidence for the existence of the rule."

However, the old meaning of prove was to test the rule. Therefore, "The exception that proves the rule." used to mean "The exception that tests (the limits of) the rule."

The meaning shifted some time during/after the renaissance, I presume, as the old meaning of prove is categorized as Middle English.

I figure you're trying to figure what other words might have meanings that change, but that is hard to tell. Spellings can change based on understanding of colloquial English, but there are hundreds of different dialects, so what sounds like how something can be spelled in one dialect can be totally different in another.

Looking to the future, we now basically have an almost universally accepted dictionary of the English language all online, so any changing to the meanings of words will come about much more slowly and likely be debated and disputed for a long time.

Hope that helps!

One of the more notorious expressions (the OP asked for a phrase) which was considered absolutely incorrect in its infancy, but has today become so widespread that many speakers are unaware that it is, semantically speaking, contradictory is

could care less

Wiktionary offers a descriptivist's approach

could care less
(idiomatic, US) Lacking interest; having apathy towards.
Clipping of couldn't care less, which is literally accurate (having no ability to care less).

Usage notes: This expression is a malapropism, since the literal meaning of this version is the opposite of the intended meaning.

Another infamous example is fewer vs. less

1. Is it ‘less items’ or ‘fewer items’?
2. Is “There were less people than I thought” unacceptable compared to “There were fewer people than I thought”?

There will be staunch defenders on both sides of the tussle. Neither are wrong. Or maybe I should I have said, neither is wrong?