I experienced a rollover error in the post count. Here is how I did it:

• Create a new moderated forum
• Post in the forum with a user requiring moderation
• Moderator deletes the post, does not approve it
• Count of posts shows 16777214

Now, that another user has posted, and been moderated, the count is now 16777215.

Using "Resynchronise post counts" in the ACP does not reset the post count.