When double-clicking the titlebar is configured in System>Preferences>Windows
to only maximize horizontally or vertically, double-clicking the titlebar in a
fully-maximized (via the button in the upper right corner) maximized window
only undoes maximization on one of the two axes, leaving the other maximized.

What's worse, the "stuck" direction now has with a border and a mouseover
mouse-pointer change that imply that manually correcting this "stuck"
maximization is possible when it isn't.  The only way to clear this up and
unglue the maximized direction is to fully maximize again, then use the window-
menu "Restore" operation or the upper-right-corner Maximize/Restore button to
undo the full maximization in both directions.