Ok, my example here is for XFree86 4.3.0, but I bet that logs have not changed
much since and that the problem still persists with the current X.org.

I always found that logs genereted by XFree86 (and X.org for the few times I had
a chance to work on more modern distributions) are not very helpfull to a normal
user.  For instance, altough I try to set my X for 1600x1200@75Hz, I only get
60Hz.  Performing "fgrep 1600 XFree86.0.log" returns:

(II) I810(0): #6: hsize: 1600  vsize 1200  refresh: 75  vid: 20393
(II) I810(0): Not using default mode "1600x1200" (bad mode
clock/interlace/doublescan)
(II) I810(0): Not using default mode "1600x1200" (bad mode
clock/interlace/doublescan)
(II) I810(0): Not using default mode "1600x1200" (bad mode
clock/interlace/doublescan)
(II) I810(0): Not using default mode "1600x1200" (bad mode
clock/interlace/doublescan)
(--) I810(0): Virtual size is 1600x1200 (pitch 2048)
(**) I810(0): *Default mode "1600x1200": 162.0 MHz, 75.0 kHz, 60.0 Hz
(II) I810(0): Modeline "1600x1200"  162.00  1600 1664 1856 2160  1200 1201 1204
1250 +hsync +vsync

For the neophyte like myself, the only thing I understand is that the chip set
supports 75Hz at that resolution (1st line), but for some reason, it settled for
60Hz.  What are the reasons?  My monitor supports 75Hz at this resolution
(tested) and I used the same monitor setting in the XFree86.conf file as the one
I had on a different machine (which has a different video card) for which
1600x1200@75Hz worked.  So its not my monitor setting that can be the cause of
the 60Hz.  So what is it?  The log file does not say it in a way that a normal
user would understand.  (BTW; if you have a solution to my problem, you can
email me personally ;) )

Notice that "Not using default mode" line appears many times.  I suspect that
these problems are the cause for the fallback to 60Mhz.  But it would be nice to
know what exactly is the problem with each of the configuration that failed. 
The message error is the same for each error and too generic.  Not helpfull.