IPhone 3GS, iOS 6.1, Serial number xx004xx… Baseband 5.16.08
Situation: I tried to downgrade my iPhone's baseband to 06.15.00 using Redsn0w. Part way through the scrolling text on the iPhone display, it stopped and said something along the lines of "ERROR! Please reboot the device into normal operating mode." This wasn't possible, however, because from this point onwards, I was restricted to either DFU mode or recovery mode.
Software I have tried to use to exit restore loop: iREB, Redsn0w, Tinyumbrella.
I also tried to restore the iPhone to the newest iOS through iTunes, but was faced with errors, 21, 11, and 1600 (when I tried it from DFU mode as a shot in the dark.)
I have changed the host file from Cydia to iTunes as suggested by other people online to no avail.
I no longer care about the baseband or jailbreak I had, but if anyone thinks they can save my iPhone, I would greatly appreciate the help.