During an internship in 1986 I wrote C code for a machine with 10-bit bytes, the BBN C/70. It was a horrible experience, and the existence of the machine in the first place was due to a cosmic accident of the negative kind.
replies(6):
9-bit bytes are pretty common in block RAM though, with the extra bit being used for either for ECC or user storage.
And I was tasked with reading a tape with binary data in 8-bit format. Hilarity ensued.