Many years ago we ordered the Intel 83C51FB masked ROM chips. Just for fun we put one of the chips in a device programmer and read in the ROM contents to the buffer, cleared a few 1's to 0's, and reprogrammed it. When that worked, we figured that Intel swallowed our mask ROM fee and programmed re-labelled 87C51FB (OTP) chips for us. We tried to order another batch of masked ROM chips, but this time we sent a hex file of all 0xFF. They wouldn't accept our order...

-- Tom Bachmann