Integer overflow

Number which is stored becomes larger than the capacity of the computer memory it is stored in. The most likely result is that the number will reset to 0 and start over.
— Wikipedia

Example:

unsigned char c = 255;
c = c - 1;
printf("%d\n", c); // 254 - OK
c = c + 2;
printf("%d\n", c); // 0 - WTF?

TODO: integer underflow