I have noticed a local dialect one round here, a double superlative. For example:
"I have the most fastest car",
Even a triple superlative : "This is my most favouritist food."
Believe me, people saying "should of" instead of "should have" or even "should've" is the minimum of worries. I have seen legally binding documents with such errors in. When you point them out to people they don't seem to understand the problem with this!
"Well, we all know what it means" is all very well, but what if some clever bu**er uses it as a get out if they don't keep to their part of the deal?! "Sorry m'lud, but this document is worth squat because this sentence makes no sense!"