Thanks to didier jodin for finding a nasty bug in Moodle 2.9.0: MDL-50373
If you have quizzes that contain randomly selected Cloze questions, then you really don't want to upgrade to 2.9 until that is fixed. (Hopefully I can get a fix done quickly and that can be included in this week's 2.9.0+ release.)
If you have not yet upgraded to 2.9, or if you do not use randomly selected Cloze questions, then there is nothing for you to worry about.
I am really sorry I introduced this bug.