2's and 3's
Working on the Dartmouth time sharing system we noticed that all input characters were converted to an internal representation and vice versa on output. So we changed the tables so 2's were converted to 3's on input and output. People would type programs with line numbers 1,2,3,4 but this was sorted to 1,3,2,4.
Our plan did have a hiccup when we mapped 2's to 3's and realized that we couldn't input a real 2 for the next mapping. So we mapped a to 2, 2 to 3, and 3 to a.