Just off the top of my head, an example of folk etymology on my part. I thought “sleight of hand” was spelled “slight”. Altho both are Scandanavian in origin and related to ’sly’, ’sleight’ refers only to a visual trick of the hand whereas ’slight’ has the sense of ’light’, ’slender’, ’unimportant’ and even ’quick’. So without noticing the spelling of the word or maybe never having seen it, I assumed the spelling ’slight’.
A question: how do you regard the usage “How was he going to know that?” in the sense of “it was not expected that he would know that and you seem to be saying it was expected of him”? Do you call it an idiom? A special meaning of ’go’? How do you categorize it?