Do you have a cubbord in your kitchen? I do. I suppose a long time ago it was a board, perhaps mounted on the wall, where cups were kept. Today we commonly speak of it as a cubbord but when we write it we always spell it "cupboard". The word has yielded to the way people want to pronounce it. Language is so very democratic that way.
In an article in Slate magazine, "Why Does Bush Go "Nucular"?", there is a discussion of "metathesis", switching of two adjacent sounds. Bush always says "nu-cu-lar" instead of "nu-clee-ar". It goes on to say, "...Bush's usage is so common that it appears in at least one dictionary. Merriam-Webster's, by far the most liberal dictionary, includes the pronunciation, though with a note identifying it as 'a pronunciation variant that occurs in educated speech but that is considered by some to be questionable or unacceptable.'"
You can often tell how fluent an English learner is by a one word answer to the question, "Do you speak English?" An intermediate level student may reply, "Yes". But often the advanced speaker will reply, "Yeah". The difference between "yes" and "yeah" can reveal fluency, familiarity and comfort with the language. I don't think any teacher teaches "yeah". I've never seen it in a coursebook. Yet if we could survey the oral English of native speakers around the world, I believe we would find it firmly established in our oral lexicon. No doubt teachers became alarmed when "yeah" began creeping into our language. I remember my English teacher forbidding us from using it despite the Beatles assuring us, "She loves you! Yeah! Yeah! Yeah!"