So, Apple have changed the behaviour on how you remove icons from the Dock.
Previously you could just drag the icon from the Dock and it would vanish in a puff of smoke. It didn’t really matter how far away from the Dock you dragged the icon.
With Mountain Lion you have to drag the icon a fair distance from the dock. You know when you have achieved this distance as the first frame of the ‘puff’ animation appears by the icon. You can then let go.
Failure to drag a sufficient distance causes the icon to jump back to the dock.
By Richard Mallion