Ooh! Excellent idea for a thread. I don't know of any that I've had that were altered, but I do recall selling a collection to someone many years ago that had some early 80s rookies in them. I told the person there were no COAs, so I couldn't guarantee authenticity.
After he bought them, he posted them for sale noting that he got them personally signed at a card show in the mid 80s.
I called him out on it, and that was that.