One of my users noticed that when she tagged a topic in a private forum, the tags nevertheless get displayed in the public tag cloud. I can think of several situations where this is potentially problematic, but in this case, it was a private discussion among site moderators about a problematic user, and the post was tagged with the user in question. So tagging is good from a moderation perspective, but probably not so good from a privacy standpoint.
For now I’ll just advise her to tag carefully, but is this a known issue? Is there any possibility it can be addressed with a hack or a change to the BP forum functions?