If you've disabled (hidden) the Prev and Next buttons on your Articulate Storyline player, you may find that they unexpectedly display when using the menu to navigate between slides.
This issue was corrected in Update 3 for Storyline. To take advantage of the new features and fixes, download and install the latest version from the link in your product confirmation email, or get it here. Then republish your project.
Note: When revisiting or reviewing question slides that have already been submitted (answered), you'll still see the Prev/Next buttons. See this article for more information.