[Galaxy S III Q&A, Help & Troubleshooting] goldcd:
How I got here:

Phone started bootlooping in my pocket/randomly locking (after occasionally a few apps crashed) within a few mins of startup.

No physical damage I'm aware of to my phone.

Tried to re-flash with Odin, which failed, and left me stuck on the "See if Kies can sort it out" message. Windows recognizes the phone on USB, but Kies never seemed to be able to connect. Download and recovery mode couldn't be loaded.

Went back to Samsung who returned both a quote for £135 repair and phone on same day - putting it down to user damage ("Octa LCD and USB Connector are damaged"). They're not. Screen works fine, USB charges and gets phone reconized by PC.

Phoned them to complain (i.e. they look fine) and eventually a month later managed to get them to take it back.

Was a very surreal conversation "We can't take it back, you broke the screen" - "No I didn't, it's on, it's next to me and doesn't have a scratch on it". "Yes sir, but my system says the screen is broken, so we can't take it back" etc etc

Came back today with "It's been tested, it's water damage and repair would exceed the cost of the handset" I'll just leave my feelings on this for now.

When the phone came back, I did suddenly realize that recovery mode now worked. Unfortunately the file system seems to be stuffed - load of errors along the lines of "E:Can't open /cache/recovery/log".

Tried to sideload via adb and got similar message that although the tranfer went fine, it couldn't be written to cache.

Likewise trying to nuke cache/return to factory settings just scroll up similar.

THEN download mode started working as well (hurrah). Tried to flash via Odin to see what would happen and got:

<ID:0/003> Odin v.3 engine (ID:3)..

<ID:0/003> File analysis..

<ID:0/003> SetupConnection..

<ID:0/003> Initialzation..

<ID:0/003> Get PIT for mapping..

<ID:0/003> Firmware update start..

<ID:0/003> boot.img

<ID:0/003> NAND Write Start!!

<ID:0/003> FAIL!

Now assuming the file system is stuffed, then that would explain this - so was wondering if I could use a PIT to just repartition the memory and work up from there - but after a bit of googling, it appears if that was required, it would be telling me it couldn't find the PIT in Odin.

Behaviour seems to have changed yet again now. Regular start takes me to the 'Download' screen (as does trying to start recovery). Download mode works fine, but I'm moving way out of my comfort zone.

Questions are:

1) Has anybody got a clue wtf might be happening? Hardware/Software? (whilst it was running for 2 minute chunks at the top, it did flag my controller as being a dodgy eMMC, but (maybe unfortunately) just assumed it was a glitch in the app meant for S2s.

2) Any advice on what to try next? Just wasted a large part of the day swearing at this and I'm still at an utter loss

3) Anybody had issues like this before with Samsung (UK). First repairer (your screen's broken) was ANovo. Second (it's a write-off) was mentioned to me on the phone, but they seem to have returned it without mentioning it and have signed off as 'Samsung' on unheaded paper with no contact details.

My next step is to crack the damn thing open to see if there is any visible water damage inside. Can't see any water indicators inside the ports - and there's a little empty square under the battery where there should be one. There should be one, I checked on another phone, just I never seemed to get one. Now just wondering if they spotted this and assumed I'd removed it (and hence blaming water damage)...

..anyway. Not sure if anybody can be bothered reading down this far - but it's been cathartic for me (and it's an HTC next time).