Images from GUI Bloopers, by Jeff Johnson. Used without permission.
What's wrong with these?
Following the above example, if an application needs to allow users to indicate that they don't want any cheese on their pizza, then the radiobutton set should include a fourth explicit value: "None", as is shown below
Alternatively, the entire setting could have an associated checkbox that, when unchecked, deactivates the radiobutton choice (i.e., causes it to be displayed with a grayed-out appearance and not be responsive) and, when checked activates it (this is recommended only when it's used consistently throughout an application).
Similarly, if a designer of an online questionanaire doesn't want to bias users' answers by picking one of the possible answers to each question as a default value, the designer should add an explicit "no opinion" value to each setting and make those the defaults..