Debian report:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=239116

Text is:
Sometimes windows will lose their focus after being horizontally maximized,
with the focus model set to Focus follows mouse. To reproduce:
Place the upper left hand corner of a window squarely over the center of
another. Right click the maximize button.
If you have Raise on focus on you will see the lower window come to the
foreground. If you don't, you'll simply the see lower window grab focus from
the window you've just horizontally maximized.

Another bug with the resizing controls is that if you maximize a window in any
way, then move it, it loses its previous notion of size entirely.
To reproduce:
Fully maximize a window then move it slightly off screen and click maximize
again.
It will move the window back to the position it had after being maximized and
won't affect the overall size.

Similarly if you only vertically maximize a window, then move it, then click
maximize again it will move the window back to its former, post-maximize
vertical location (horizontal remains unchanged), but the size remains the
same.

-- end bug --

I can reproduce these on 4.0.3 too.