I have a MBP13 so I keep the dock hidden for screen space reasons. So, when in full screen mode for apps (like Safari) sometimes the dock would pop up, others times it wouldn't. To get the dock to pop up when in a full screen app use one finger to swipe the mouse arrow to the bottom of the screen like you normally do then... simply do it again. I just figured it out... kind of a "duh" moment. At first I thought it was a bug but it's by design so one can use the mouse arrow at the bottom without having the dock get in the way.