A common misconception among idiots who go by their ears rather than their brain. The real way to say it is "Could HAVE", i.e. "I could HAVE gone to the party", rather than "I could OF gone to the party".It's a common mistake, generally caused because the phrase "Could have" is said so quickly that it is conceived as " Could of ", but that's wrong. Should of " is the exact same but slightly less common mistake.