https://bugs.freedesktop.org/show_bug.cgi?id=69790
--- Comment #2 from Ville Syrjala ville.syrjala@linux.intel.com --- I managed to reproduce this on my HSW. But it seems that it's somehow random as it doesn't happen every time. It's rather hard to reproduce actually when you just retry the same mode w/ 'testdisplay -o'.
There's no difference in dmesg between a good and bad modesets using the same mode.
As it stands, I don't have any good ideas as to what might be happening.