In comments, when someone's username is mentioned in comments (e.g., @applemonkey496), it does not seem to give any indication that the mention worked, and that the user was notified. Furthermore, if someone's username is spelled incorrectly and therefore doesn't work, there is no indication of that either. For example, "@applemonkey495" will be styled the same as "@applemonkey496" even though one of them works and the other does not.
When a user is mentioned, the name could be written in bold or even linking to the user's profile:
@applemonkey496 or @applemonkey496