I knew I should have saved the link to the post, I can't find it now. It wasn't in the FAQ forum. I thought the post was about milky oil but it disappeared. If it happens again I'll save the link to the post.
This one is explainable. If you are replying to a post or thread and while you are writing a reply the thread is moved or deleted or locked - Bingo. It's because your reply has no where to go.