Just a note that may save somebody else some debugging time:
Symptoms: Build and debug (RAM) worked great. Build and debug (flash) suddenly failed, every time, with a verification error. The mis-matches were all in the first two pages (0x0000 - 0x01FF) of flash.
My assumption was that I had either accidentally changed something in a linker option or in the system startup file, or that the J-Link had finally quit.
After poking and prodding at various possibilities, it turns out that I had lost the pull-up on the BSL pin. The bootloader was, therefore, taking over that area, just as it was supposed to. Obvious in hindsight ...
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