People usually set argument and often don't mind about validation. Just leave basic validation thinking most will pass. But that is not the case. For example you can use User ID ad argument and think both UID and User Name would pass, but that is not the case. You have to go and select "User" Validator and use "Allow both numeric UIDs and string usernames" for this to work. Otherwise you could have a view that is showing nothing and left you wondering what is happening.