If a tag is copied and pasted, it does not alert user.
If you type in @Freeze, it would alert the user. But if you copy and paste the tag (say from another post) the @tag doesn't alert the user, since the username is considered a "hyperlink" and it doesn't turn into the user BB code that is produced from tagging.
1. Tag a user. Check the tagged account for alert.
2. Copy and paste the tag, user is not alerted for that tag.
Clicking "edit" on the 2nd message also shows that the tags aren't converted to the user BBCode.
This issue came up in a thread where the thread starter would require to tag (and alert) a set of users frequently, the thread starter kept copying and pasting the tags until the users reported not being alerted by the tags. Sometimes, if multiple users need to be tagged frequently (with the lack of group tagging) people usually copy/paste.