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I'm guessing you were looking for a case where the placement of the adjective causes ambiguity. In practice it's not so common, though it can happen e.g.:
"un jeune africain" = "an African youth"
"un jeune Africain" = "a young African"
(Of course, we're talking about a slight difference of emphasis -- essentially both phrases mean pretty much the same thing anyway.)
The word "youth" can be used as a collective noun, or to refer to an individual.
So "youth" can be "la jeunesse", "les jeunes" in general. But "a youth" is "un(e) jeune".
Here, the problem is not the placement of the adjective, since in the first case "jeune" is the name and "africain" the adjective, whereas in the second case, "jeune" is the adjective and "Africain" the name.
In any case, when the adjective refers to the subject or the complement, you use the adjective form ("She drives fast cars", fast refers to cars -> "elle conduit des voitures rapides").
When the adjective refers to the action, you use the adverbial form ("She drives cars fast", fast refers to drives -> "elle conduit des voitures rapidement").