It could of been different.
Uhhnnn. (That's the sound of the incorrect buzzer going off.)
"Could of" is wrong, wrong, wrong. As is, "would of" and "should of."
My guess is this mistake originated from the contractions "could've/would've/should've" which sound like "could of/would of/should of" when spoken aloud.
- Correct: It could have been different.
- Correct: I would have eaten the pie but I was too full.
- Correct: You should have gone to the movie with us.