[duckman]

“Because of its exclusive self-checking features...”

I am from the 2nd generation hardware era as well. Started with IBM in the late 60’s. Coded in 1410/7010 Autocoder, precursor to Assembler, no self correction as far as I knew.

[Kirkwood]

I think this refers to the use of a compiler.

[bigbob]

Jeez, I thought that with all the computer geeks here everyone was aware of what UNIVAC was referring to in this ad!

PARITY!

The Excess 3 code included a parity bit, which was used for error checking. This technique was quite new, having been first introduced in 1951 on magnetic tape drives:

http://www.computer-history.info/Page4.dir/pages/Univac.dir/

[Old North State]

Just a hunch, but I imagine the ‘never makes a mistake’ statement was a garbled attempt by marketing people to describe the use of parity bits in magnetic core memory. It was kind of a big deal back in the old days and improved reliability, but difficult to explain to non-technical people.