http://en.wikipedia.org/wiki/Welsh_Not
A bit extreme as an example, as that's not even an issue of dialects. But even in England (and other English speaking areas) the idea that there is a correct form of English and that dialects were corruptions of something pure was a major part of the education system until quite recently. It's not uncommon to find people (usually over fifty) who switch their accents and grammar when talking to someone in a formal setting. And, obviously, it hasn't totally disappeared.
Here's a current example:
http://www.sheffieldtelegraph.co.uk/news/local/accent_on_pupils_talking_properly_1_4266351
Poll
