This really applies to other ROMs as well, but the "new" JM6/A/C/D ROMs specifically.
Some of these ROMs include new bootloaders. These bootloaders check checksums/signatures in various parts of the firmwares. The "normal" Samsung ROMs, nor custom ROMs and kernels, have these checksums.
The result is that once flashed, you cannot revert to older/official/custom Samsung ROMs, and you are pretty much stuck using one of these four ROMs, as they are the only ones containing the right checksums.
At least TRIPLE CHECK if you want to flash one of these ROMs, that what you are flashing DOES NOT include the new bootloaders ( boot.bin and sbl.bin ). I know from the CF-Root thread that a fair number of you are already too late, but I thought to warn new users anyways. Some modders (like rotohammer) already usually remove these parts, but still triple check everything to make sure.
There is no known fix. I know, I've tried all of them some people suggested in other threads. None of them really works. Sure, with some effort, you can get a different firmware to somewhat run, but you'll still be using the "checksum" bootloaders and the kernel will not be modified. You will still be running the kernel from the "checksum" firmware you loaded earlier. You will not be able to do full flashes, nor will KIES updates work.
Hopefully somebody will find a real solution for this issue for those already affected. If so, please post it in this thread.