Unfortunately, because it is not a widely recognised phenomenon, it is often disregarded as "an error".
What is this phenomenon? It's that in some languages, case marking only occurs when there is a single "head noun" in the noun phrase -- introduce "and" or "or" and the marking is lost.
The classic example (and this is where most of you will cry "that's wrong") is in English pronouns. "Me and him went to the cinema." "Him and me don't talk anymore." Yes, there are some people who will say "He and I" for both those cases, but then a lot of them tend to hypercorrect and say things like "between you and I", which puts the nominative form where the oblique form would be expected in the classical model. I'm sure there are some people who consistently mark cases in their pronouns within complex noun phrases; but it is observable fact that a great many native speakers of English do not.
And so I propose making a distinction in case marked languages that only mark case in simple, one head-noun noun phrases, which I call "simplex case marking languages", and those that maintain case marking in longer noun phrases, which I call "complex case marking languages".
Enough about English for the moment, let's look at the Romance language family. Most Romance languages retain complex case marking in pronouns, even though most have lost noun marking.
So Spanish, for example, will use "yo y tú" as subject if strictly required, and "a mí y a ti" as object. This gives us things like yo y tú no tenemos nada en común -- literally "I and you have nothing in common". But French doesn't do this. French is an example of a simplex case marking language. And yet it tries its hardest not to be. The closest equivalent of the Spanish sentence above would be toi et moi, nous n'avons rien en commun (I think) -- literally "you and me, we have nothing in common". So the complex subject is relegated to a tag prefixing the main clause, and a simplex pronoun that is properly case marked. As for as I know, though, you'll never find a case marked complex noun phrase in French.
And this even carries through in the absence of a verb. eg
¿Quién viene mañana? - Who's coming tomorrow?
Whereas the French answer would either be:
Moi, je viens
or more likely simply
And in English, we have a similar choice: I am (with the verb) or just me.
Even among those who still support "you and I" style English, there are very few (I hope!) who would try to enforce a distinction between not I and not me. Modern English really always comes down to not me. French sticks with moi (me), whereas Spanish will use yo (I) or a mí (me) according to case.
¿Quién lo tiene? - Qui l'a? - Who has it?
yo no - Moi, non. - Not me.
This leaves me wondering: is there an intrinsic link between simplex case marking and the lack of case marking in verbless utterances? Or do the similarities in pattern between French and English simply further contact effects caused by the English and French history of mutual invasion?
Not quite the same... or is it?
A similar phenonomenon can be seen with Scottish Gaelic. The Celtic languages have an interesting feature -- prepositions have merged with the following pronoun and altered over time, so eadar (between) + sinn (us) = eadarainn. But this only holds for a simplex nounphrase. I want to say "between us and them", it wouldn't be "*
But Gaelic's in a funny situation compared to French and English. Fused forms aside, the pronouns appear to have less case marking than common nouns. Gaelic still has a three way distinction of sorts: a "default" (for lack of a better word), a genitive and a dative. But many people will argue that this is actually archaic and doesn't reflect real usage -- case marking in Gaelic is a massive bone of contention, and it's something that won't be sorted out until a proper corpus is assembled.
But I think one of the stumbling blocks in the current academic discussion is the failure to take into account simplex and complex case marking. The handling of prepositional pronouns does suggest that the Gaelic speaker's internal model deals with complex and simplex noun phrases separately. But more than this, William Lamb's PhD corpus, though small, contained enough for him to say fairly confidently that the dative tends only to be marked when the preposition rules a single noun -- no ands or ors, and no genitival components.
But that would leave Gaelic in a funny state. A genitive can only, by its nature, appear in a complex noun-phrase, so the division of simplex vs complex is just a little to simple to hold the key here....