After the latest Ubuntu upgrade my mouse became extremely unstable.  I use Gnome 3 and at random times, my mouse’s X and Y values would jump to 0.  This made it difficult to complete simple tasks as well as the annoyance of Gnome’s hot window from the top left corner being triggered constantly just by moving the mouse anywhere in the screen.  A bit of research led me to an Arch Linux thread where others were experiencing the same issue.  The solution turned out to be simple!:

xinput set-prop "VirtualBox mouse integration" "Coordinate Transformation Matrix" 0.5 0 0 0 0.5 0 0 0 1.0
xinput set-prop "VirtualBox mouse integration" "Coordinate Transformation Matrix" 1.0 0 0 0 1.0 0 0 0 1.0

More information can be found at the thread where I found the result, https://bbs.archlinux.org/viewtopic.php?pid=1150418#p1150418