We’ve had this a few times with posts. Spammers will go into an existing topic and copy and paste another post and reply with it, sometimes adding a spammy link, sometimes not (just to build up trust, I suppose?).
These will tend to get held in the moderation queue because the spammer will have “typed” too fast, but it can be difficult for mods to tell that it’s spam without going into the topic and scrolling up a bit.
If there were a indicator like “this post is 95% similar to another post in the topic” that would be 