After report this issue to sprockets, I feel like maybe I should report it here, since it happen after I upgraded Rails. But to avoid duplicated information, please check it on sstephenson/sprockets#270 for details.
Have you tested using rails 3-2-stable branch and sass-rails 3-2-stable branch?, this was fixed there.
Thanks for reporting, @rainux! See #4151 for the fix.
Use rails 3-2-stable branch and sass-rails 3-2-stable branch can fix this issue, thanks!