There are 2 answers to this question: Exponential degrade or similar effect to this one
Both have usernames specified, but they have the same icon and no links to profiles:
What does this mean?
It means that the user name is not associated with an account on the site. Most often, this occurs when the question was migrated from another Stack Exchange site, but the user does not have an account here to link to. On occasion, a user account can be deleted, but that is much less common.