Good reminder. Not specific to MySQL, though. This happens in many languages that have unsigned integers. The underlying bit pattern resulting from twos-complement arithmetic is the same for signed and unsigned arithmetic, but when interpreted as unsigned you get one more high bit instead of that bit representing the sign: 0 for positive, 1 for negative.

Shouldn't that be "0 - 1"?

"1 - 1" will correctly be computed as "0".