I recently had a problem with a friend's S3. They had rooted their SGS3, which invalidates the warranty and is clearly something I would never do myself (ahem)!
The phone had been worked fine with its rooted OS (CF-Root-SGS3-v6.4.zip) for months, but on the evening of Saturday 4th May, Vodafone pushed an update. That update seemed to conflict with the rooted OS, and when they returned to the phone it was stuck in an endless loop; it would flash the Samsung Galaxy S3 screen (white text on black background) for a few seconds, then go black (reset), then flash it again for a few seconds.
My first thought was "Vodafone bricked my (friend's) S3". Fortunately the skills I/he/she had learned rooting it came in handy for rescuing it.
I read some good posts
found a good (by which I mean standard and apparently reliable) S3 ROM
installed the Samsung Mobile USB drives
and used Odin 1.85 to transfer it to my phone
From there I reconfigured the phone and reinstalled its apps. I then installed Samsung Kies and used it to install the latest (official) ROM:
Now I was almost out of the woods, but then my phone started freezing up again. Turns out this is a known issue:
I installed Dummy File Generator
which took an hour or so to write noise across my internal and external (MicroSD card) phone memory. Since then, the phone's been working perfectly. I think the moral of the story (for my friend of course) is never to root or jailbreak a device, but I'm pleased to get it working properly again.