I have one that has bothered me for about twenty years, yet has never been explained. I even did a research paper on it in college (my B.A. is in English).
In the town of Webster, MA, and seemingly only in said town, the word "sherbet" is mispronounced as "sure, Bert."
To clarify, I am referring to the frozen confectionary treat, often sold in fruit flavors such as orange. Nearly 100 percent of individuals I have known in my life pronounce it correctly. However a handful of people I've known from Webster, MA, for some reason, make it sound like they are giving an affirmative to a friend named Bert. Their relatives and neighbors did the same.
For the record, I have never lived in or close to Webster, but knew a few people who were raised there. When visiting Webster on numerous occasions and ordering sherbet at the Friendly's restaurant there, I was blown away how the population of the town had a universal mispronunciation. I first became aware of this when ordering sherbet and listening to the employees there.
Webster enjoyed a huge influx of Polish immigrants for labor purposes 100 years ago. My initial theory that those folks may have contributed was dismissed.
The school system, and perhaps teachers and administrators who came from within or attended the same schools, did not appear to play a major role. The mispronunciation spanned generations and the school faculty was a mixed bag of ages and backgrounds.
My final theory, which was neither proven nor discounted, was that it was cultural due to a local business. There were two private ice cream stands that sold sherbet between the 1960s and 1990s. It is possible that one or both of them had the word spelled wrong on a sign, or just one employee at a super popular destination was pronouncing it wrong, and the sheer popularity of those places led to the mispronunciation spreading like wildfire in the community. The one strange element to that explanation is that, quite literally, outside the town limits of Webster one does not encounter the mispronunciation. I would imagine that residents of other towns would have visited the ice cream shops and then have adopted the mispronunciation.