Some users have reported that plugins like Jerome's Keywords and UTW and other postmeta adding plugins have been deleting their postmeta values for a post when a comment is left on that post.
I installed Jerome's Keywords and experienced the behavior. The problem is this:
- Jerome's Keywords always assumes that it was able to successfully insert its field into the edit form.
- If the edit_post hook is called in a circumstance where the plugin was NOT able to insert its field into the edit form, it assumes that the user wanted to delete all keywords. Thus, the plugin deletes all the keywords.
- Commenting in 2.1 calls wp_update_comment_count(), which runs the edit_post hook.
While plugin authors are definitely to blame for assuming that "doesn't exist" is the same as "exists and is empty," we might want to consider using an alternative hook for announcing that the comment count has been updated for a post.