[AT&T, Rogers, Bell, Telus Samsung Galaxy S III] -Mr. X-:
How to UNBRICK:

Credit goes to these guys in this thread, I thought i was onto something origional, however after doign some searching i found others have figured this out for a different Variant of the S3 :)

What you did:

Bricked downgrading the bootloader after upgrading to the 4.3 bootloader. The 4.3 bootloader appears to blow efuses that stop previous boot loaders from booting. Now your EMMC has non booting garbage on/in it. YAY! not...

What we are doing:

What we will be doing is writing the Partition layout (Partition.mbn) among other things to the SD so the phone will boot from it after it fails to boot from the EMMC. According to my research the CPU is designed to cascaded from one storage device to the other in the event of failure on the first device. This also means if your EMMC is smoked, you could technically run the whole rom and everything from the sd card, or that's my assumption.

What you need:

- A Micro SD card 16GB or larger. this will no work on smaller cards, i tried a 2GB 4 GB and 8GB for the sake of testing and it fails.

- A working Dump that includes the SBL1, SBL2, SBL3, aboot, TZ and RPM from a working device that matches your DEVICE MODLE ie: I747U or I747M If you wanto t create your own dump from a friends device the command in a rooted adb shell would be as follows but not exact depending where you want the dump file to end up being on your sd cards. "dd if=/dev/block/mmcblk0 of=/storage/sdcard1/debrick.bin bs=1024 count=81920" That is what I used my my co-workers CM 10.2 rom to get the 4.3 bootloader/debrick dump. NOTE: the device has to be rooted with a working busybox that supports the dd command.

- The ability to write the dump image to the micro sdcard.

What to do:

First:

I used "Win32DiskImage" Located here But you can also use dd to write the image to your sd card in Linux.

On Windows:

Open Win32DiskImage. Brows to the Debrick dump you have for your device. Select the drive letter if your micro sd in the drop-down, click write. Bam done.

On Linux:

For ease of example, and method. Open Gparted on your Linux install or live cd, check the dev your sd card is in the drop down menu. For example mine was /dev/sdb/ then use "dd if=/path/to/debrick/dump.img of=/dev/sdb/" This method also works. Be careful you are selecting the right dev or you will overwrite another disk on your system. YOU HAVE BEEN WARNED.

Second:

Put that SD card in your device insert the battery and if the SD card is written correctly with a proper debrick file. You can put a jig in the device and it will boot straight to download mode. If you dont have a jig hold Vol Down + Home + Power the device will boot to download. If it does not, either the image didn't write to the card properly ( I had to write it a few times with Win32DiskImager, DD worked fine the first try), the debrick dump is no good. Or your trying to debrink your 4.3 bootloader downgrade brick with a dump from 4.1.2 compatible bootloaders. this will not work the blown efuse will still prevent the bootloader from booting from the SD card as well as the EMMC. You will know pretty fast if it worked, as the device will vibe with the jig or it will vibe with the button combo almost immediately after being pressed and held.

Third:

Odin that thing back to the 4.3 stock rom. Yes you have to. You ain't getting 4.1.2 bootloaders back so dont think about odining that thing either. it wont boot, with out the SD with the 4.3 bootloader on it installed. Odining back is the best thing to do, it will get the right boot loader on your EMMC. If you are feeling adventurous and you had a custom recovery installed. You can boot to recover instead of download from the debricking SD boot, and install a 4.3 flashable zip bootloader there, if all goes well, you wont lose your data and the bootloader will be functional on the EMMC. Either method you choose after its done "flashing" what ever method. remove the sd and verify it boots on its own.

Note: a user stated that he flashed a bootloader via recovery and flashable zip, and he stated it appeared to flash the bootloadeerr to the micro sd. if this happens, ODIN back to the proper rom to get the internal EMMC rebuilt and booting.

If it does, do a dance or something.

Here is a flashable for the i747M 4.3 Bootloader

Here is a debrick Dump from a I747M from the UEMK5 4.3 that works

Here is an untested debrick dump for I747M from the DMF1 4.1.2 (WOuld be used to debrick a device that was bricked before or that has not updated to 4.3 bootloader only)

Here is a debrick file from the I747U on 4.3 thanks to theramsey3 I cannot test it obviously but it looks good in a hex editor. Should work for any one who has bricked the I747U by downgrading the boot loader.