Questions of type Multiple-Choice, True-False, or Multi-Select must be carefully answered for the responses to register.
If a student taking a quiz clicks a radio button or clicks a check box to specify or modify an answer to a Multiple-Choice, True-False, or Multi-Select Question, the 'click' does not take unless the mouse button is released before the mouse is moved off the radio button or check box. A student feeling pressed for time may perceive a question of one of these types may have been answered or modified and go on to the next question, not knowing that the response didn't take.
This is a known issue.
Students should be advised to make sure their answers are visibly selected by a dot for radio button questions or a check for check box questions.
Note that for un-answered questions, the student will receive a warning when attempting to submit. However, no warning would be issued if the student did not successfully modify an initial response. For example, the student initially selects choice b in a multiple choice with choices a,b,c,d and saves the answer, later comes back to the question and realizes choice c is correct, but quickly clicks c, doesn't lift the mouse button before moving the mouse, scrolls on, and mistakenly thinks the change to the correct answer has been made.
On some browsers, selecting a radio button or check box and moving the mouse before releasing the button will leave a box or dashed marquee around the control.