[Xperia SP Q&A, Help & Troubleshooting] Coover55:
Here's the log from one of my latest attempts to flash my device.

26/053/2016 13:53:06 - INFO - Selected Bundle for Sony Xperia SP(C5303). FW release : 12.1.A.1.207. Customization : AU

26/053/2016 13:53:06 - INFO - Preparing files for flashing

26/053/2016 13:53:19 - INFO - Please connect your device into flashmode.

26/053/2016 13:53:30 - INFO - Device connected in flash mode

26/053/2016 13:53:30 - INFO - Opening device for R/W

26/053/2016 13:53:30 - INFO - Reading device information

26/053/2016 13:53:30 - INFO - Phone ready for flashmode operations.

26/053/2016 13:53:30 - INFO - Opening TA partition 2

26/053/2016 13:53:30 - INFO - Current device : C5306 - YT91096HPS - 1271-7717_R4B - 1269-3538_12.1.A.1.205 - TELCEL-LTE_12.1.A.1.205

26/053/2016 13:53:30 - INFO - Closing TA partition

26/053/2016 13:53:30 - INFO - Start Flashing

26/053/2016 13:53:30 - INFO - Processing loader.sin

26/053/2016 13:53:30 - INFO - Checking header

26/053/2016 13:53:30 - INFO - Flashing data

26/053/2016 13:53:32 - INFO - Loader : S1_Root_7054 - Version : R5G006 / Boot version : R11B023 / Bootloader status : UNROOTABLE

26/053/2016 13:53:32 - INFO - Max packet size set to 64K

26/053/2016 13:53:32 - INFO - USB buffer size set to 512K

26/053/2016 13:53:32 - INFO - No flash script found. Using 0.9.18 flash engine

26/053/2016 13:53:32 - INFO - Opening TA partition 2

26/053/2016 13:53:32 - INFO - Writing TA unit 00002774. Value : 01

26/053/2016 13:53:32 - INFO - Closing TA partition

26/053/2016 13:53:32 - INFO - Processing partition-image.sin

26/053/2016 13:53:32 - INFO - Checking header

26/053/2016 13:53:32 - INFO - Flashing data

26/053/2016 13:53:32 - INFO - Opening TA partition 2

26/053/2016 13:53:32 - INFO - Closing TA partition

26/053/2016 13:53:32 - INFO - Opening TA partition 2

26/053/2016 13:53:32 - INFO - Processing amss_fsg.sin

26/053/2016 13:53:32 - INFO - Checking header

26/053/2016 13:53:32 - INFO - Flashing data

26/053/2016 13:53:33 - INFO - Processing amss_fs_1.sin

26/053/2016 13:53:33 - INFO - Checking header

26/053/2016 13:53:33 - INFO - Flashing data

26/053/2016 13:53:33 - INFO - Processing amss_fs_2.sin

26/053/2016 13:53:33 - INFO - Checking header

26/053/2016 13:53:33 - INFO - Flashing data

26/053/2016 13:53:33 - INFO - Processing cache.sin

26/053/2016 13:53:33 - INFO - Checking header

26/053/2016 13:53:33 - INFO - Flashing data

26/053/2016 13:53:36 - INFO - Processing fotakernel.sin

26/053/2016 13:53:36 - INFO - Checking header

26/053/2016 13:53:36 - INFO - Flashing data

26/053/2016 13:53:38 - INFO - Processing kernel.sin

26/053/2016 13:53:38 - INFO - Checking header

26/053/2016 13:53:38 - INFO - Flashing data

26/053/2016 13:53:40 - INFO - Closing TA partition

26/053/2016 13:53:40 - INFO - Opening TA partition 2

26/053/2016 13:53:40 - INFO - Processing system.sin

26/053/2016 13:53:40 - INFO - Checking header

26/053/2016 13:53:42 - ERROR - Processing of system.sin finished with errors.

26/053/2016 13:53:42 - INFO - Ending flash session

26/053/2016 13:53:42 - ERROR -

26/053/2016 13:53:42 - ERROR - Error flashing. Aborted

26/053/2016 13:53:42 - INFO - Flashing finished.

26/053/2016 13:53:42 - INFO - Please unplug and start your phone

26/053/2016 13:53:42 - INFO - For flashtool, Unknown Sources and Debugging must be checked in phone settings

26/053/2016 13:53:42 - INFO - Device connected in flash mode

26/053/2016 13:53:43 - INFO - Device disconnected

26/053/2016 13:53:45 - INFO - Device connected in flash mode

26/054/2016 13:54:28 - INFO - Device disconnected

26/055/2016 13:55:33 - INFO - Device connected in flash mode

26/055/2016 13:55:54 - INFO - Device disconnected