Re: @ Andrew Jones 2
"You did, I assume, attend school?"
IIRC the Oxford Dictionary people are predicting that "should of" will enter the English language as a normal construct in the not too distant future.
The "correct" English language at any time is often determined by common mis-usage. Words have changed to mean the opposite of the original. It is now apparently generally accepted (not by me) that "decimate" equals "annihilate" - rather than the less severe "kill one in ten".
IIRC the spelling of "could" was introduced by a monk because he incorrectly assumed that "coud" had lost the "l" - and by similarity of usage needed to match the spelling of "should".