1. Menu items. Pull down the main menu on OSX, and find a menu with a separator - a line between groups of related actions. The separator itself isn't a target - it doesn't lead anywhere - but clicking it dismisses the menu. So, miss your intended target by a few pixels, and frustration ensues.
2. Window edges. This one is a side-effect of OSX's poor support for full-screen windows. Even if you 'maximise' a window, moving your pointer to an extreme edge does, for example, allow you to move the scrollbar, but persists in offering the 'size window' action. So you need to go all the way to one edge, then move ever so slightly back again, which reduces the target area from infinite to very small. Full-screen addresses this, whilst introducing other 'flaws' (like your global menubar no longer being visible).