Excess-3 code was used on some older computers as well as in cash registers and hand-held portable electronic calculators of the 1970s, among other uses.
Excess-3 arithmetic uses different algorithms than normal non-biased BCD or binary positional system numbers.
[1] Also, when the sum of two excess-3 digits is greater than 9, the carry bit of a 4-bit adder will be set high.
Because a 4-bit integer can only hold values 0 to 15, an excess of 6 means that any sum over 9 will overflow (produce a carry-out).
A fault in a memory or basic transmission line may result in these codes.