When I use @username in a comment, sometimes it is deleted when I post, and deleted again when I try to edit it back in.
A case in point is @mac in this thread:
How to fix a racked/sagging exterior wood entry door?
Does this reflect a setting that the user has configured on their account? or ...?
This is automatically done by the server when the @name is redundant, such as when you try to reference the OP in a comment (though once there are a few others in the comment stream, it's no longer redundant and the server once again allows the @name). They will still get the notification in these situations. You will know this is going to happen when @name doesn't autocomplete for you.
For the full details, see How do comment replies work?
