At least this is what I found happened to mine. I remember Tiggy saying we lost a lot of posts after the last forum upgrade. I was looking through the FAQ and it was all facked up. I hit modify and found this symbol � in place of ' and sometimes just randomly placed. Because of it you couldn't read the post, although it was still there.
It seemed to only happen to my posts that is because I usually compose my posts in Word and then copy paste them into the Org. If I click modify on the post and replace all the � then the posts reappear.