I think that "could of" was probably derived from the use of "could've" being short for "could have". Unless one knows how it is spelled, it could sound like "could of".
And little 5yr old granddaughter insists on using "of" instead of "if". She has been to day care and kindy so can only imagine she has heard other kids doing this as she has never heard it at home or here.
Maybe that's the next generation's contribution to the misuse of "of"...lol
