Explain it please.
there are MANY threads about this.
basically, the process of "writing" flash memory is turning ones into zeros, whereas "erasing" is turning zeros into one.
you can "overwrite" zeros with zeros as much as you like and you can "overwrite" ones with ones as much as you like.
Then, we have chips with ECC memory. If your chip has ECC memory you can't do this. To understand why, go to the chip product page -> documents -> family reference manual, the chapter on flash with ECC.
long story short, there is a parity word that is calculated every 48bits of flash, which is stored along with the 48 bits. When you change a bit a new parity word will be calculated and there is good chance that is going to be different.. let's say 0x70 is the old and 0x1F is the new... but you can't turn 0 into F by writing so the actual value will be 0x10.
Next time the flash controller will have to read the two flash words an ECC trap will be generated and if no handler is present there will be a reset.
the ECC is reset with an erase so
-Overwrite the OTP and the OTP will be useless (can't erase it)
-Overwrite the configuration words and you won't be able to start the device, at all. It can also be very tricky to enter programming mode to erase the device, so you may just want to toss it.
If you still have questions.. there are MANY threads where this was explained in much more detail, then there's also the documentation.