horizontally maximised can get stuck to the edge of the screen so that you can't move them sideways by dragging the title bar. minimising and maximising seems to fix this.

i have seen it happen with epiphany, gedit and nautilus, so i assume it is happening at the metacity layer.

the easiest way to reproduce is with gedit.
open a text file
resize the window so that it touches both sides of the screen, but not the top and bottom.
close the document
open it again, it should open at them same size
now try to move the window by dragging the title bar

this is with metacity 2.18.2-0ubuntu1

this is also reported at https://bugs.launchpad.net/ubuntu/+source/metacity/+bug/106749