Never did see a similar question, nor did I experience such a hidden button at all. Are you sure there is no back button on a previous slide which is set to display for the rest of the project? Master slide used for KC slides is exactly the same as for a quiz slide. I did explore the differences between KC and quiz slides in depth in a blog post, but since I never had the issue you describe, it probably will not be of any help. I tend to UNcheck the option 'Allow Backward movement' in Quiz Preferences. That is valid for both types of slides.