I'm too lazy to go turn on showdead for GP, but...

> I don't know pixie_ but non-native English speakers hear "could of" and "would of" when we use the contractions "could've" and "would've".

You actually hear "could ov" and "would ov", but since the latter aren't actually words, there's an automatic shunt to the nearest known word. Sometimes this occurs in the speaker; sometimes this occurs in the listener. It occurs in the speaker because of loopback: you say "could ov"; you hear "could of"; you say "could of".