Iris

A column that fills itself also removes the whole class of bug where somebody adds a row and the formula does not reach it.

Wrote a 400-character formula that replaced eleven helper columns and I am the person the second comment is warning about.

It worked perfectly and I could not debug it. Six months later a source column changed and I rewrote the whole thing rather than find the fault, because there was no intermediate value to look at anywhere.

The helper columns were the debugger and I had deleted it.