In Norway, Valdresmål and Bokmål are very different, perhaps comparable to the difference between Norwegian and Swedish. Most people, linguist and non-linguist alike, consider Norwegian and Swedish to be different language, although there is a minor (dismissive) sub-meme in linguistics that consider them to be dialects of Norwedish (a.k.a Swegian). Most linguists have no opinion about Valdresmål and Bokmål as language vs. dialect. I don't know if anyone has conducted a scientific survey, but I think most Norwegians consider them to be "dialects" of Norwegian.
The general rule is that most linguists are maximal splitters, so if it's reasonable to call A and B separate languages, they will do so. But we are largely led by local sentiments, to the extent that we know anything about such sentiments. This can lead to a historical progression of terminology, where categorization changes when we learn more. An example is Dogri, an Indic language of Jammu, which was considered to be a dialect of Punjabi. After a bit of real research on the language, it became apparent that it has to be treated as a separate language. Similarly, Hawrami has been treated as a dialect of Kurdish (this is an ethnic issue), but the linguistic evidence suggests (I'll remain neutral on the correctness of the conclusion) that it is not a dialect of Kurdish, it is on a separate branch of Northwestern Iranian. Similarly, Karaga, Kalanga, Ndau, Manyika, Korekore and Zezuru have been considered to be "dialects of Shona", but now the received opinion is that they are mostly if not all separate, related languages.
In other words, there are numerous cases where it is assumed that two speech forms are a single language and if one is identifiable (has a name), it is a dialect, but deeper digging reveals that the assumed language is really more than one language. SIL invented a term to cover this problem: macrolanguage. Thus what was once the "Luhya language" is not the Luhya macrolanguage, containing many specific languages (ones formerly termed "dialects").