No, you shouldn’t freak out over this, but it is always good to be prepared for the worse! There’s a huge discussion going on both on Reddit and XDA Developers where owners of Samsung Galaxy S III devices discuss their bad luck: phones are dying for no reason.
To be specific: “Tl;Dr – S3 mainboards dying for no reason, Samsung replacing mainboards for free, but no new hardware revision so it might just happen again“, says a post on Reddit. There are also other speculations too mentioning a probable fault with the phone’s NAND memory that is giving up.
So far there is no official information or acknowledgment from Samsung about the severity or frequency of this problem. Reports are claiming, as you can read in both the sources below, that Samsung might be aware of the issue and replacing/repairing units free of charge, even if tampered with (root, custom ROMs, etc.). However, these are just suppositions, just like those saying the replaced components will likely give up after a while too.
That being said, all of our Samsung Galaxy S III units, international or US, as well as those of people we know, are still doing fine, knock on wood, and we hope they’ll stay that way!