'Automatically hide and show the Dock' is ticked in the dock preferences pane, but the dock never autohides.
Perhaps relevant, tooltips for application names also hang about indefinitely after the mouse pointer has left the dock area - below is a screenshot of the dock with a 'Calendar' name tooltip showing. The tooltip does not go away unless the mouse pointer is hovered over the dock again and then smoothly moved out of the dock area, so as to not leave any tooltips showing.
I don't usually set the dock to autohide, it is an attempt to get around a bug (perhaps related, perhaps not) where the dock continues to always show even when playing a video in fullscreen with VLC. I've already tried
killall Dock - the dock did not relaunch and I had to restart the computer. I also deleted a preference file of some sort under
~/Library/Application Support named something like
com.apple.Dock or similar - unfortunately I've forgotten the exact name.