I am forwarding the following bug from a Ubuntu reporter:
https://bugs.launchpad.net/ubuntu/+source/xorg/+bug/211098

The user has a Gateway EV910 monitor (edid "name" gwy232a) which can be DDC probed properly but produces EDID that both read-edid and xorg fail to understand:

http://launchpadlibrarian.net/13079404/ddcprobe.txt
http://launchpadlibrarian.net/13079429/edid.txt
http://launchpadlibrarian.net/13079409/Xorg.0.log

From the read-edid output, it sounds like the monitor has some extended data in it?

	...
	Call failed

 The EDID data should not be trusted as the VBE call failed
 EDID claims 32 more blocks left

Xorg's output follows:

(WW) NVIDIA(GPU-0): The EDID read for display device CRT-0 is invalid: the
(WW) NVIDIA(GPU-0):     checksum for EDID version 1 is invalid.
(--) NVIDIA(GPU-0): 
(--) NVIDIA(GPU-0): Raw EDID bytes:
(--) NVIDIA(GPU-0): 
(--) NVIDIA(GPU-0):   00 ff ff ff ff ff ff 00  ff ff 2c 23 b6 b0 01 00
(--) NVIDIA(GPU-0):   2a 0b 01 03 1c 24 1b aa  e9 68 45 a3 53 47 9a 24
(--) NVIDIA(GPU-0):   10 48 4c ff ff 80 61 59  45 59 31 59 61 40 31 4f
(--) NVIDIA(GPU-0):   3b ca 81 99 a9 4f 1a 4f  40 30 62 b0 32 40 40 c0
(--) NVIDIA(GPU-0):   10 00 5e 06 11 00 00 1e  00 00 00 fd 00 32 a0 1e
(--) NVIDIA(GPU-0):   5f 14 00 0a 20 20 20 20  20 20 00 00 00 fc 00 47
(--) NVIDIA(GPU-0):   61 74 65 77 61 79 20 45  56 39 31 30 00 00 00 fc
(--) NVIDIA(GPU-0):   00 0a 20 20 20 20 20 20  20 20 20 20 20 20 00 5b
(--) NVIDIA(GPU-0): 
(II) NVIDIA(0): NVIDIA GPU GeForce 6600 (NV43) at PCI:1:0:0 (GPU-0)
(--) NVIDIA(0): Memory: 262144 kBytes
(--) NVIDIA(0): VideoBIOS: 05.43.02.46.01
(II) NVIDIA(0): Detected AGP rate: 8X
(--) NVIDIA(0): Interlaced video modes are supported on this GPU
(--) NVIDIA(0): Connected display device(s) on GeForce 6600 at PCI:1:0:0:
(--) NVIDIA(0):     CRT-0
(--) NVIDIA(0): CRT-0: 400.0 MHz maximum pixel clock
(II) NVIDIA(0): Assigned Display Device: CRT-0
(==) NVIDIA(0): 
(==) NVIDIA(0): No modes were requested; the default mode "nvidia-auto-select"
(==) NVIDIA(0):     will be used as the requested mode.
(==) NVIDIA(0): 
(II) NVIDIA(0): Validated modes:
(II) NVIDIA(0):     "nvidia-auto-select"
(II) NVIDIA(0): Virtual screen size determined to be 640 x 480
(WW) NVIDIA(0): Unable to get display device CRT-0's EDID; cannot compute DPI
(WW) NVIDIA(0):     from CRT-0's EDID.
(==) NVIDIA(0): DPI set to (75, 75); computed from built-in default