Developer and Inventor of CF-Root Kernels “Chainfire” Confirmed over Twitter that the infamous “eMMC bug” also referred to as the “ICS super brick bug” has been re-introduced with the latest ICS rom on Galaxy SII. The latest revision of Android leaked to the Galaxy SII, Ice Cream Sandwich 4.0.4 XXLQ5 is the firmware currently confirmed as known to cause this bug. Users are advised not to upgrade to this firmware, as this may cause permanent damage to the eMMC area of your device.
For those who are unfamiliar with the eMMC bug, It is a critical bug that has been identified with Ice Cream Sandwich firmwares which will lockup a part of the memory accessible to a user on eMMC when a Factory Data wipe command is issued. The wipe command, regardless of the method it is carried out (Stock or Over Clockworkmod) will trigger an area of the eMMC to lockup causing the device to freeze at the boot logo during the immediate next bootup. The two most common devices that was affected with this bug is the Samsung Galaxy SII (GT-i9100) and the Samsung Galaxy Note (GT-N7000).
There are various methods to revive the device from this hardbrick state back to usable condition at the cost of a few gigabyte from the internal memory, However a completely successful method to reverse the damage and to revive a bricked device back to factory condition has not been identified yet.
Users who are already on the above mentioned firmware must take caution not to make Factory Data wipes over Settings or over Clockworkmod until a method to avoid this is developed. Users who are awaiting to update to the Ice Cream Sandwich 4.0.4 XXLQ5 firmware are advised not to upgrade until the matter is resolved.