I expirenced this on another forum, its when there was an error in the email address of a user, and that user had set to recieve a reply when someone replied in that topic.
As far as I'm aware you get one email from when the topic is first posted in to when you next visit - so not every time someone replies. It is useful in things like support forums that you don't visit very often and you post a request and only want to visit again when someone replies.
Deadly wrote:
Topher wrote:Stuff about Thatcher....
You are a disgrace and I'm looking forward to when someone you respect dies so I can rub your liberal face in it.