Occasionally, a post is deleted if it is part of a thread with a not permitted subject line. That could be anything but defamation or heavy flaming is at the top.
Moderators can only delete one post at a time or the whole thread. Obviously, one can't expect moderators to work 24 hours a day although a few posters tend to think they should. Deletion of several posts in a thread can be time consuming.
Posters don't check what the subject line was and use it regardless.
I can recall a thread where a prime minister was marginalized in the heading, so the whole thread disappeared regardless of some very good posts being automatically deleted as well.
Some posters will try anything hoping they can get away with it and if the thread is deleted, blame the moderator.
The message is: if the subject line is not acceptable, please start a new thread.
