First, let me thank you for your question. Second, let me suggest that, even though featuring the focus of your question in the subject heading of the thread is very helpful, it would also be nice if you would also include the phrases in the body of your post.
I like all of the examples that Gustavo and David have suggested, but their explanations all seem to be missing something.
First, I find the phrase "in haste" to be borderline archaic. I'm trying to think whether I've ever heard it outside of the context of the movie Monty Python and the Holy Grail.
On the other hand, the phrase "in a hurry" is quite common in modern parlance. However, despite the presence of a singular indefinite article, which implies that "hurry" is a discrete countable noun, I don't believe I've ever heard it used in the plural:
1: *They are all in hurries.
These sentences all mean pretty much the same thing:
2a: They knew they were in serious trouble.
2b: They knew they were in a big fix.
2c: They knew they were in dire straits.
There is no way to change the pronouns or anything else in these sentences to make "big fixes" make sense in (2b) or "a dire strait" in (2c).
I suggest that these are set idiomatic phrases that defy logical explanation, as do many other aspects of the English language.
I hate this answer, and I will be very happy if someone can show me where I'm wrong.
All the blessings of the new year.