The most common method to check for a constituent being logged in looks like this:
[[?x[[S1:user_name]]x::xx::
The x's located in the above conditional runs an exact match on the [[S1:username]].

Translated, that appends an x to the beginning and end of the user name. If the string returned by the conditional has two x's next to each other (i.e. "xx"), the user is not considered to be logged in. This can cause issues if a user name begins or ends with an x. If I have a user name "xuser", appending an x at the beginning and end will result in "xxuserx" and a false positive will result because the string begins with "xx".

If this scenario arises, you will need to create a more complex conditional that has a lower likelihood of conflicting with a user name. For example, try appending "xyz" at the beginning and end of the user name and then check for the existence of "xyzxyz".