Playing with xrandr on my beat-up old laptop (Trident Cyberblade card, lcd
hard-locked at 1024x768 resolution only)

I start out in 1024x768x75 and then see what else is available:

[kaboom@skuld kaboom]$ xrandr -q
 SZ:    Pixels          Physical       Refresh
*0   1024 x 768    ( 347mm x 260mm )  *75   70   60  
 1    800 x 600    ( 347mm x 260mm )   75   72   60   56  
 2    640 x 480    ( 347mm x 260mm )   75   72   60  
 3    832 x 624    ( 347mm x 260mm )   74  
Current rotation - normal
Current reflection - none
Rotations possible - normal 
Reflections possible - none
[kaboom@skuld kaboom]$ 

So then I switch to a lower resolution (xrandr -s 1). It correctly shrinks.
Since my lcd is hardlocked at 1024x768, it just displays as an 800x600 window w/
a 1024x768 border (as expected). It doesn't center the 800x600 w/in the 1024x768
frame though -- instead it anchers the 800x600 display at (0,0). No big deal,
just different than what I might have expected / what usually happens when
running lower res on a fixed res lcd....

At any rate, I then switch back (xrandr -s 0). It appears to go back to
1024x768, but the desktop is really still only 800x600.  The last 224 pixels
(from 576 to 800) are simply copied and displayed again as pixels 801 - 1024.
For example, if I move my mouse to about (700,10), it shows up as (700,10) and
as (924,10). Ditto for dialog boxes, stuff in the gnome panel at the bottom of
the screen, etc.

To make things weirder:

* its just the horizontal stretching that's screwed. Vertically, everything's
fine (going from 600 to 768 actually did go to 768, rather than copying the
bottom 168 pixels and displaying them twice)
* I tried using the gimp to do a screen capture of the effect. In the screen
capture, everything's completely normal

If I flip to a text VC and then back to X, everything straightens out, and I get
real 1024 resolution