This is happening in full screen secure mode during quiz attempt (tested on IE8/Firefox/Chrome). When the student makes left click / right click anywhere other than the appropriate place(s) in the full screen window, the attached window pops up and does not go by selecting OK. Also, the instructions to test taker are not clear like do not click elsewhere except at appropriate place(s).

Refer: http://moodle.org/mod/forum/discuss.php?d=165772 .