I have a setup with two screens (same size) with the primary screen being on the left, and 
the secondary on the right. I put my panel vertically on the right hand side of the left hand side screen 
-- see also attached screenshot of primary screen.
In this setup, if I maximize a window, it maximizes to the full screen size, and goes therefore below
the vertical panel, with the panel hiding some part of the maximized window. The panel setting 
does not allow this (you could configure there to produce this behaviour, but this is not activated -- 
the panel is set always visible). 

I guess this is related to the fact that I have right to the panel the second screen configured, I use a similar panel setup on my single screen laptop setup, and there the maximized window  respects the panel.

I could see this behaviour through several versions of kwin & plasma, the current version string kwin
reports, is:
Qt: 4.8.6
KDE: 4.14.2
KWin: 4.11.12

all of that on openSUSE 13.2 (currently) but was also on openSUSE 13.1 and through several versions of the kde workspace and kwin and plasma.

Reproducible: Always

Steps to Reproduce:
1.Put panel vertically on the right hand side of the left screen of a two screen setup
2. Maximize a Window on the left screen, on which the panel is.
3. Recognize that the maximized window goes below the panel, being maximized to the full
   physical size of the screen and some part therefore being hidden below the panel

Actual Results:  
The maximized window goes below the panel, being maximized to the full
   physical size of the screen and some part therefore being hidden below the panel

Expected Results:  
The maximized window should only use the space not already covered by the panel.