Have you ever had one of those weird posts on a phpbb2 forum where somebody just quotes what someone else wrote and then writes nothing in return. I always that it was just some idiot fulling around with our boards but it turns out, the source of these ghost posts is improper indexing in Google.
Google will index phpbb2 urls with "posting.php?mode=quote" embedded in the url. The first time I saw this I nearly dropped a brick. How could Google know what somebody was about to submit? But it turns out they didn't but there is an indexing problem. When one of these urls gets in the serps and somebody clicks on it. If the next thing the click on is submit, they will send a ghost post to your forum.